_______________General Description
The MAX777L/MAX778L/MAX779L are pulse-skipping DC-DC converters that step up from low-voltage inputs (1V guaran-teed). They require only three external components—an induc-tor (typically 22μH) and two capacitors. The MAX777L delivers a 5V output, the MAX778L generates pin-selectable voltages of 3.0V or 3.3V, and the MAX779L output can be adjusted from 2.5V to 6V through an external resistive divider.
The devices include an Active Rectifier TM that eliminates the need for an external catch diode, and permits regulation even when the input is greater than the output. Also, unlike those in other step-up converters, the MAX777L/MAX778L/MAX779L’s Active Rectifier TM turns off in the shutdown mode, disconnect-ing the output from the source. This eliminates the current drain associated with conventional step-up converters when off or in shutdown.
High-frequency operation (up to 150kHz) allows the use of small, surface-mount inductors with values of 10μH or less.Supply current is 190μA under no load and only 20μA in stand-by mode; supply voltage can range from 1V to 4.5V (1 to 3cells). With a 2V input, the devices typically deliver 200mA at 5V, or 300mA at 3V.
For fully specified devices designed for step-up/step-down applications (where the input can be above or below the out-put), refer to the MAX877L/MAX878L/MAX879L data sheet.
________________________Applications
Single Battery-Cell (1V), Step-Up Voltage Conversion Efficient, High-Power Step-Up Regulation from Low Input Voltages Pagers
Portable Instruments & Hand-Held Terminals Notebook and Palmtop Computers
____________________________Features
o 1V to 4.5V Input Guarantees Start-Up Under Load o Up to 210mA Output
o Load Fully Disconnected in Shutdown o 82% Efficiency
o Output in Regulation with Input Voltage above Output Voltage o Internal 1A Power Switch and Active Rectifier o Adjustable Current Limit Allows Low-Cost Inductors o 190μA No Load Supply Current o 20μA Shutdown Supply Current o 3V/3.3V (MAX778L), 5V (MAX777L), and Adjustable (MAX779L) Output Voltage
______________Ordering Information
Ordering Information continued at end of data sheet.* Dice are specified at T A = +25°C, DC parameters only.
**Contact factory for availability and processing to MIL-STD-883.
MAX777L/MAX778L/MAX779L
Low-Voltage Input, 3V/3.3V/5V/
________________________________________________________________Maxim Integrated Products
1
__________________Pin Configuration
__________Typical Operating Circuit
For free samples & the latest literature: https://www.wendangku.net/doc/a518197696.html,, or phone 1-800-998-8800
Active Rectifier is a trademark of Maxim Integrated Products.
M A X 777L /M A X 778L /M A X 779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters 2_______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(V IN = +2.5V, I LOAD = 0mA, L = 22μH, C OUT = 100μF, SHDN and ILIM connected to IN, AGND connected to PGND, T A = T MIN to T MAX ,typical values are at T A = +25°C, unless otherwise noted.)Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage (IN to PGND).......................................0V to 4.5V Output Short-Circuit Duration to PGND, AGND (Note 1)....30sec Voltage Applied to:
LX (switch off).......................................................-0.3V to 4.5V (switch on)....................................30sec short to IN or OUT OUT, SHDN ...........................................................-0.3V to +7V FB ...........................................................-0.3V to (OUT + 0.3V)AGND to PGND........................................................-0.3V, +0.3V Reverse Battery Current....................................................900mA
Continuous Power Dissipation (T A = +70°C)
Plastic DIP (derate 9.09mW/°C above +70°C).............727mW SO (derate 5.88mW/°C above +70°C)..........................471mW CERDIP (derate 8.00mW/°C above +70°C)..................640mW Operating Temperature Ranges:
MAX77_LC_A.......................................................0°C to +70°C MAX77_LE_A....................................................-40°C to +85°C MAX77_LMJA.................................................-55°C to +125°C Storage Temperature Range.............................-65°C to +150°C Lead Temperature (soldering, 10sec).............................+300°C
Note 1:The output may be shorted to ground if the package power dissipation is not exceeded.
MAX777L/MAX778L/MAX779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters
_______________________________________________________________________________________3
Note 2:Output in regulation, V OUT = V OUT (nominal) ±4%.
Note 3:At high V IN to V OUT differentials, the maximum load current is limited by the maximum allowable power dissipation in the
package (see Absolute Maximum Ratings ).
Note 4:Minimum value is production tested. Maximum value is guaranteed by design and is not production tested.
Note 5:V OUT is set to a target value of +5V by 0.1% external feedback resistors. V OUT is measured to be 5V ±2.5% to guarantee the
error comparator trip point.
Note 6:Start-Up guaranteed under these load conditions.
ELECTRICAL CHARACTERISTICS (continued)
(V IN = +2.5V, I LOAD = 0mA, L = 22μH, C OUT = 100μF, SHDN and ILIM connected to IN, AGND connected to PGND, T A = T MIN to T MAX ,typical values are at T A = +25°C, unless otherwise noted.)
M A X 777L /M A X 778L /M A X 779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters 4_______________________________________________________________________________________
__________________________________________Typical Operating Characteristics
(Typical Operating Circuit, T A = +25°C, unless otherwise noted).
501
2
5
SHUTDOWN SUPPLY CURRENT
vs. INPUT VOLTAGE AND TEMPERATURE
60
INPUT VOLTAGE (V)S H U T D O W N S U P P L Y C U R R E N T (μA )
70803
4
4010203007
6
2.0
1
2
5
MAX778L
SHDN THRESHOLD VOLTAGE
vs. INPUT VOLTAGE AND TEMPERATURE
2.5INPUT VOLTAGE (V)
S H D N T H R E S H O L D V O L T A G E (V )
3.03.53
4
1.50.5
1.07
6
90
400.1
1
1000
MAX777L/MAX779L
EFFICIENCY vs. LOAD CURRENT
50
LOAD CURRENT (mA)E F F I C I E N C Y (%)
60708010
100
3090
400.1
1
1000
MAX778L
EFFICIENCY vs. LOAD CURRENT
50LOAD CURRENT (mA)
E F F I C I E N C Y (%)
6070
8010
100
30300
500.5
1.0
2.5MAX777L/MAX779L
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
100INPUT VOLTAGE (V)O U T P U T C U R R E N T (m A )
150200250 1.5 2.00350
500.5
1.0MAX778L
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
100INPUT VOLTAGE (V)
O U T P U T C U R R E N T (m A )
150200250 1.5 2.0
3003002001
2
5
NO-LOAD SUPPLY CURRENT vs. VOLTAGE AND TEMPERATURE
220SUPPLY VOLTAGE (V)N O -L O A D S U P P L Y C U R R E N T (μA )
2402602803
4
180120140160100
7
6
MAX777L/MAX778L/MAX779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters
_______________________________________________________________________________________5
A: I OUT , 200mA/div, 0mA to 200mA B: V OUT , 50mV/div, AC COUPLED MAX778L, V OUT = 3.3V, V IN = 2.5V
LOAD-TRANSIENT RESPONSE
A
B
2ms/div
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, T A = +25°C, unless otherwise noted.)
A: V IN , 1V/div, 1.8V to 3.3V
B: V OUT , 100mV/div, AC-COUPLED, I OUT = 240mA MAX778L, V OUT = 3.3V
LINE-TRANSIENT RESPONSE
A
B
2ms/div
A: SWITCH VOLTAGE (LX PIN), 2V/div B: INDUCTOR CURRENT, 0.5A/div
C: OUTPUT VOLTAGE RIPPLE, 50mV/div, AC COUPLED MAX777L, V IN = 1.5V, I OUT = 100mA
SWITCHING WAVEFORMS, CONTINUOUS CONDUCTION
A
B C
0mA 5μs/div
A: SWITCH VOLTAGE (LX PIN), 2V/div
B: INDUCTOR CURRENT, 0.5A/div
C: OUTPUT VOLTAGE RIPPLE, 50mV/div, AC COUPLED
MAX777L, V IN = 3V, I OUT = 70mA
SWITCHING WAVEFORMS, DISCONTINUOUS CONDUCTION
A
B C
2μs/div
M A X 777L /M A X 778L /M A X 779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters 6
_______________________________________________________________________________________
A: SHDN, 2V/div B: V OUT , 1V/div V OUT = 3V
MAX778L START-UP TIME
A
B
10ms/div
_____________________________Typical Operating Characteristics (continued)
(Circuit of Figure 1, T A = +25°C, unless otherwise noted.)
NAME FUNCTION
1
ILIM Sets switch current-limit input. Connect to IN for 1A current limit. A resistor from ILIM to IN sets lower peak inductor currents.2IN Input from battery
3AGND Analog ground. Not internally connected to PGND.
PIN
4PGND Power ground. Must be low impedance; solder directly to ground plane or star ground. Connect to AGND, close to the device.
5LX Collector of 1A NPN power switch and emitter of Active Rectifier PNP.6OUT Voltage output. Connect filter capacitor close to pin.
7
SHDN Shutdown input disables power supply when low. Also disconnects load from input. Threshold is set at V IN /2.
8
N.C.(MAX777L)
No connect—not internally connected.
SEL (MAX778L)Selects the main output voltage. 3.3V when hard-wired to AGND, 3.0V when left open.
______________________________________________________________Pin Description
FB (MAX779L)
Feedback input for adjustable-output operation. Connect to an external voltage divider between V OUT and AGND.
_______________Detailed Description
Operating Principle
The MAX777L/MAX778L/MAX779L combine a switch-mode regulator with an NPN bipolar switch, current limit, precision voltage reference, and active rectifier—all in a single monolithic device. In shutdown mode,the internal rectifier is completely turned off and dis-connects the load from the source. Only two external components are required in addition to the input bypass capacitor: a 22μH inductor and a 100μF filter capacitor.
A minimum off-time, current-limited, pulse-frequency-modulation (PFM) control scheme combines the advan-tages of pulse width modulation (PWM) (high output power and efficiency) with those of a traditional PFM pulse skipper (low quiescent currents).
External conditions (inductor value, load, and input volt-age) determine the way the converter operates, as follows:
At light loads,the current through the inductor starts at zero, rises to a peak value, and drops down to zero in each cycle (discontinuous-conduction mode). In this case, the switching frequency is governed by a pair of one-shots that set a maximum on-time inversely propor-tional to V IN [t ON = 8.8/(V IN - 0.25)] and a minimum off-time (1.3μs for MAX777L/MAX779L and 2.3μs for MAX778L). With a 22μH inductor, LX’s peak current is about 400mA and is independent of input voltage.Efficiency at light loads is improved because of lower peak currents.
At very light loads , more energy is stored in the coil than is required by the load in each cycle. The converter regulates by skipping entire cycles. Efficiency is typically 65% to 75% in the pulse-skipping mode. Pulse-skipping waveforms can be irregular, and the output waveform contains a low-frequency component. Larger, low equiv-alent series resistance (ESR) filter capacitors can help reduce the ripple voltage if needed.
MAX777L/MAX778L/MAX779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters
_______________________________________________________________________________________7
Figure 1. MAX778L Block Diagram
M A X 777L /M A X 778L /M A X 779L
At heavy loads above approximately 100mA, the con-verter enters continuous-conduction mode, where cur-rent always flows in the inductor. The switch-on state is controlled cycle-by-cycle by either the maximum t ON time or the switch’s preset current limit. As a result, the switch's current rating is not exceeded and the inductor is not saturated. At very heavy loads, the inductor cur-rent self-oscillates between this peak current limit and some lower value governed by the minimum off-time,the inductance value, and the input/output differential. With ILIM shorted to IN, the peak switch current of the internal NPN power switch is set to 1A. The peak switch current can be set to a lower value by connecting a resistor between ILIM and IN (see Current Limit sec-tion). This enables the use of physically smaller induc-tors with lower saturation-current ratings. At 1A, the switch voltage drop (V SW ) is about 500mV. V SW decreases to about 250mV at 0.1A.
Conventional PWM converters generate constant-fre-quency switching noise, while this architecture pro-duces variable-frequency switching noise. However,the noise does not exceed the current limit times the fil-ter-capacitor ESR, unlike conventional pulse-skippers.
Step-Down Mode
If the input voltage exceeds the output voltage, the MAX777L/MAX778L/MAX779L behave as “switched”linear regulators. If the output voltage starts to drop, the switch turns on and energy is stored in the coil, as in normal step-up mode. After the switch turns off, the voltage at LX flies high. The active rectifier turns on when LX rises above V IN . As in a linear regulator, the voltage difference between V IN and V OUT appears across the rectifier (actually a PNP transistor) until the current goes to zero and the rectifier turns off. At high V IN to V OUT differentials, the maximum load current is limited by the maximum allowable power dissipation in the package. For fully specified buck/boost converters,refer to the data sheet for the pin-compatible MAX877L/MAX878L/MAX879L.
Active Rectifier
The internal active rectifier of the MAX777L/MAX778L/MAX779L replaces the external Schottky catch diode in normal boost operation. The rectifier consists of a PNP pass transistor and a unique control circuit which, in shutdown mode, entirely disconnects the load from the source. This is a distinct advantage over standard boost topologies, since it prevents battery drain in shut-down.
The active rectifier also acts as a zero-dropout regulator if the input exceeds the regulated output. This allows the MAX777L/MAX778L/MAX779L to act as buck/boost
converters. Useful in battery-powered applications,where the battery voltage may initially exceed the output voltage, the converters will regulate down to the output voltage and seamlessly switch into boost mode as the input drops below the output voltage. The pin-compati-ble MAX877L/MAX878L/MAX879L are fully specified buck/boost converters with higher specified output cur-rents than the MAX777L/MAX778L/MAX779L.
Shutdown
Shutdown (SHDN ) is a high-impedance, active-low input. Connect SHDN to V IN for normal operation.Keeping SHDN at ground holds the converters in shut-down mode. Since the active rectifier is turned off in shutdown mode, the path from input to load is cut, and the output effectively drops to 0V. The supply current in the shutdown state ranges from 4μA at V IN = 1V to 50μA at V IN = 4.5V. The shutdown circuit threshold is set nominally to V IN /2 + 250mV. When SHDN is below this threshold, the device is shut down and is enabled with SHDN above the threshold. When driven from external logic, SHDN can be driven to a higher voltage than V IN .
Current Limit
Connecting ILIM to IN sets an LX current limit of 1A. For smaller output power levels that do not require the max-imum peak current, the peak inductor current can be reduced to optimize overall efficiency and to allow very small, low-cost coils with lower current ratings. See also the Inductor Selection section.
Reduce the MAX777L/MAX778L/MAX779L peak induc-tor current by connecting a resistor between ILIM and IN. See Figure 2 to select the resistor.
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters 8
_______________________________________________________________________________________
Figure 2. Current-Limit Resistor vs. Current Limit
The output voltage of the MAX777L is fixed at 5V. The MAX778L output voltage can be set to 3V by leaving the SEL pin open. Connect SEL to AGND for 3.3V operation. The MAX779L’s output voltage is set by two resistors, R1 and R2 (Figure 3), which form a voltage divider between the output and the FB pin. The output voltage can be set from 2.5V to 6.0V by the equation:
V OUT= (0.2025) [(R1 + R2)/R2]
To simplify the resistor selection:
R1 = (R2)[(V OUT/0.2025) - 1]
Since the input current at FB is 40nA maximum, large val-ues (10k?to 50k?for R2) can be used with no significant loss of accuracy. For 1% error, the current through R2 should be at least 100 times FB’s bias current.
When large values are used for the feedback resistors (R1 > 50k?), stray output impedance at FB can add “lag” to the feedback response, destabilizing the regula-tor and creating a larger ripple at the output. Lead lengths and circuit board traces at the FB node should be kept short. Reduce ripple by adding a “lead” compensa-tion capacitor (C3, 100pF to 50nF) in parallel with R1.
__________Applications Information The Typical Operating Circuit shows a MAX777L step-up application circuit. This circuit starts up and oper-ates with inputs ranging from 1.0V to 4.5V. Start-up time is a function of the load, typically less than 5ms. Output current capability is a function of the input voltage. See Typical Operating Characteristics.
Inductor Selection
The 22μH inductor shown in the Typical Operating Circuit is sufficient for most MAX777L/MAX778L/
MAX779L designs. Other inductor values ranging from
10μH to 47μH are also suitable. The inductor should
have a saturation rating equal to or greater than the
peak switch-current limit, which is 1A without an exter-
nal current limit (ILIM connected to IN). It is acceptable
to operate the inductor at 120% of its saturation rating; however, this will reduce efficiency. For highest effi-ciency, use an inductor with a low DC resistance, preferably under 0.2?. Table 1 lists suggested inductor suppliers.
Capacitor Selection
The 100μF, 10V surface-mount tantalum (SMT) output capacitor shown in the Typical Operating Circuit will provide a 20mV output ripple or less, stepping up from
2V to 3.3V at 200mA. Smaller capacitors, down to 10μF,
are acceptable for light loads or in applications that tol-
erate higher output ripple. The input capacitor may be omitted if the input lead length is less than 2 inches
(5cm) or if the loads are small.
The primary factor in selecting both the output and
input filter capacitor is low ESR. The ESR of both bypass and filter capacitors affects efficiency. Optimize performance by increasing filter capacitors or using specialized low-ESR capacitors. The smallest low-ESR
SMT tantalum capacitors currently available are Sprague 595D or 695D series. Sanyo OS-CON organic semiconductor through-hole capacitors also exhibit
very low ESR, are rated for the wide temperature range,
and are particularly useful for operation at cold temper-atures. Table 1 lists suggested capacitor suppliers.
Layout
The MAX777L/MAX778L/MAX779L’s high peak currents
and high-frequency operation make PC layout impor-
tant for minimum ground bounce and noise. Locate
input bypass and output filter capacitors close to the device pins. All connections to the FB pin (MAX779L) should also be kept as short as possible. A ground
plane is recommended. Solder AGND (pin 3) and PGND (pin 4), directly to the ground plane. Refer to the
MAX777L/MAX778L/MAX779L evaluation kit (EV kit) manual for a suggested surface-mount layout. MAX777L/MAX778L/MAX779L
Low-Voltage Input, 3V/3.3V/5V/ Adjustable Output, Step-Up DC-DC Converters
_______________________________________________________________________________________9
M A X 777L /M A X 778L /M A X 779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters 10______________________________________________________________________________________
Table 1. Component Suppliers
PRODUCTION METHOD
INDUCTORS
CAPACITORS
Miniature
Through-Hole
Sumida
RCH654-220
Low-Cost Through-Hole Nichicon
PL series low-ESR electrolytic United Chemi-Con,LXF series
Surface Mount
Sprague 595D Sprague 695D
Matsuo 267 series AVX
TPS series
Sumida
CD54-220 (22μH)Murata-Erie
LQHYN1501K04M00-D5(15μH)Coiltronics CTX20-1
Coilcraft PCH-27-223
Sanyo OS-CON
low-ESR organic semiconductor AVX USA:(803) 946-0690
(800) 282-4975, FAX (803) 626-3123Coiltronics USA:(561) 241-7876, FAX (561) 241-9339Matsuo USA:(714) 969-2491, FAX (714) 960-6492Murata-Erie USA:(814) 237-1431
(800) 831-9172, FAX (814) 238-0490Nichicon USA:(847) 843-7500
Sanyo USA:(619) 661-6835, FAX (847) 843-2798Japan:(81) 7-2070-6306, FAX (81) 7-2070-1174Sprague USA:(603) 224-1961, FAX (603) 224-1430Sumida
USA:(847) 956-0666, FAX (847) 956-0702Japan:(81) 3-3607-5111, FAX (81) 3-3607-5144United Chemi-Con
USA:(714) 255-9500, FAX (714) 255-9400
___________________Chip Topography
MAX777L/MAX778L/MAX779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters
______________________________________________________________________________________11
TRANSISTOR COUNT: 170; SUBSTRATE CONNECTED TO AGND.
SHDN
OUT
PGND
LX 0.084" (2.134mm)
0.068" (1.727mm)
_Ordering Information (continued)
PART TEMP. RANGE PIN-PACKAGE MAX778L CPA 0°C to +70°C 8 Plastic DIP MAX778LCSA 0°C to +70°C 8 SO MAX778LC/D 0°C to +70°C Dice*
MAX778LMJA -55°C to +125°C 8 CERDIP**MAX778LESA -40°C to +85°C 8 SO
MAX778LEPA -40°C to +85°C 8 Plastic DIP MAX779L CPA 8 Plastic DIP MAX779LCSA 0°C to +70°C 8 SO MAX779LC/D 0°C to +70°C Dice*
MAX779LEPA -40°C to +85°C 8 Plastic DIP MAX779LESA -40°C to +85°C 8 SO
MAX779LMJA
-55°C to +125°C
8 CERDIP**
0°C to +70°C * Dice are specified at T A = +25°C, DC parameters only.
**Contact factory for availability and processing to MIL-STD-883.
M A X 777L /M A X 778L /M A X 779L
Low-Voltage Input, 3V/3.3V/5V/
Adjustable Output, Step-Up DC-DC Converters ________________________________________________________Package Information
12______________________________________________________________________________________