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KTG8502

流降至１ｕＡ的．该芯片封装Ｓ　Ｏ　Ｐ　－　８　和

集成１３０欧姆功率ＭＯＳ开关的输出可调　

２．１Ａ输出电流宽４．７５Ｖ到２５Ｖ工作电压　

并　提供连续负载电流２Ａ以上４．７５Ｖ至２５Ｖ该器件集成了两个１３０　欧　姆ＭＯＳＦＥＴｓ，３４０ｋＨｚ频率逐周期过流保护输入欠压锁定　

+12V

０．９２５Ｖ至１９ＶＵｐ至９３％的效率可编程　

软启动稳定低ＥＳＲ陶瓷输出电容器固定　ＳＯＰ－８和ＥＳＯＰ－８封装说明

特点

宽输入电压　。电流模式控制提供快速瞬态响应和电流限制。可调软启动可防止浪涌电流导通，并在关断模式下电源电

ＥＳＯＰ－８，　可提供了最少的外部元件非常紧凑的解决方案。

应用

分布式电源系统网络系统　ＦＰＧＡ，ＤＳＰ，ＡＳＩＣ电源供应器　绿色电子产品／设备笔记本电脑典型应用

KT G 8502

PACKAGE REFERENCE

PIN FUNCTIONS

1IN 2SW 3GND

COMP

6EN 7SS

...... 4.75V to 25V

ABSOLUTE MAXIMUM RATINGS

Supply Voltage V IN ...............................................................................-0.3V to +26V

Switch Node Voltage V SW .................................................................................... 27V Boost Voltage V BS .................................................................V SW –0.3V to V SW + 6V All Other Pins...........................................................................................-0.3V to +6V Junction Temperature ..................................................................................... +150°C Lead Temperature .................................................................................... +260°C/10s Storage Temperature .................................................................... –55°C to + 150°C

Recommended Operating Conditions

Input Voltage (V IN ) ......................................................................... Output Voltage (V SW

) ............................................................................ 0.925 to 20V Operating Temperature.......................................................................–20°C to +85°C

Functional Block Diagram

2A.26V Synchronous Step-Down Converter

KTG8502

ELECTRICAL CHARACTERISTICS (V IN = 12V, T A = +25°C, unless otherwise noted.)

Note1:Guaranteed by design, not tested.

25 uses current KTG 8502 is a synchronous rectified,

OPERATION FUNCTIONAL DESCRIPTION

The current-mode, step-down regulator. It

regulates input voltages from 4.75V to V down to an output voltage as low as 0.925V, and supplies up to 2A of load current. The -mode control to regulate the output voltage. The output voltage is measured at FB through a resistive voltage divider and amplified through the internal transconductance error amplifier. The voltage at the COMP pin is compared to the switch current measured internally to control the output voltage. The converter uses internal N-Channel MOSFET switches to step-down the input voltage to the regulated output voltage. Since the high side MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between SW and BS is needed to drive the high side gate. The boost capacitor is charged from the internal 5V rail when SW is low.

When the FB pin exceeds 20% of the nominal regulation voltage of 0.925V, the over voltage comparator is tripped and the COMP pin and the SS pin are discharged to GND, forcing the high-side switch off.

APPLICATIONS INFORMATION

COMPONENT SELECTION

Setting the Output Voltage

The output voltage is set using a resistive voltage divider from the output voltage to FB pin.The voltage divider divides the output voltage down to the feedback voltage

by the ratio:

V FB =R2

R1+R2

×V OUT

Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is:

V out =R2+R1

×0.925

Inductor

The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower

saturation current. A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by:

L =Vout Fs×?I L

×(1?D) D =

Vout Vin

;

Where V OUT is the output voltage, V IN is the input voltage, F S is the switching

KTG 8502

frequency, and ΔI L is the peak-to-peak inductor ripple current.

Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by:

I LP=I LOAD+Vout×(1?D)

Where I LOAD is the load current.

The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirements.

Optional Schottky Diode

During the transition between high-side switch and low-side switch, the body diode of the low side power MOSFET conducts the inductor current. The forward voltage of this body diode is high. An optional Schottky diode may be paralleled between the SW pin and GND pin to improve overall efficiency.

Input Capacitor

The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. Choose X5R or X7R dielectrics when using ceramic capacitors.

Since the input capacitor absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by:

I Cin=I LOAD×?D×(1?D)

The worst-case condition occurs at V IN≥2V OUT, where I Cin≥I LOAD/2. For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1μF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple for low ESR capacitors can be estimated by:

?V IN=I load C in×F S×D×(1?D)

Output Capacitor

The output capacitor is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by:

?V OUT=V OUT L×F S×(1?D)×(R ESR+18×F S×C OUT)

In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the

employs current mode control for easy can be optimized for a wide range of

capacitance. For simplification, the output voltage ripple can be estimated by:

?V OUT =V OUT

8×L ×Fs 2×C OUT

×(1?D)

In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to:

V OUT =V OUT

L ×F S

×(1?D)×R ESR

The characteristics of the output capacitor also affect the stability of the regulation system. The capacitance and ESR values.

Compensation Components

compensation and fast transient response. The system stability and transient response are controlled through the

COMP pin. COMP pin is the output of the internal transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system.

The DC gain of the voltage feedback loop is given by:

A VDC =R LOAD ×G CS ×A VEA ×V FB

OUT

Where Av EA is the error amplifier voltage gain; G CS is the current sense transcend- uctance and R LOAD is the load resistor value.

The system has two poles of importance. One is due to the compensation capacitor and the output resistor of the error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at:

F P1=

G EA

COMP VEA

F P2=1

OUT LOAD

Where G EA is the error amplifier transconductance.

The system has one zero of importance, due to the compensation capacitor and the compensation resistor. This zero is located at:

F z1=

COMP COMP

The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, due to the ESR and capacitance of the output capacitor, is located at:

F ESR =1

2π×C OUT ×R ESR

In this case (as shown in Figure 2), a third pole set by the compensation capacitor (C6)

KTG 8502KTG 8502

and the compensation resistor (R3) is used to compensate the effect of the ESR zero on the loop gain. This pole is located at:

F p3=163

The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where the feedback loop has the unity gain is important. Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system instability.

A good rule of thumb is to set the crossover frequency below one-tenth of the switching frequency.

To optimize the compensation components, the following procedure can be used.

1.Choose the compensation resistor to set the desired crossover frequency.

Determine the R3 value by the following equation:

R3=2π×C OUT×F C

EA CS×V OUT

FB<

2π×C OUT×0.1×F S

EA CS×

V OUT

Where F C is the desired crossover frequency which is typically below one tenth of the switching frequency.

2. Choose the compensation capacitor to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, F Z1, below one-forth of the crossover frequency provides sufficient phase margin.

Determine the compensation capacitor value by the following equation:

C COMP>4COMP C

3.Determine if the second compensation capacitor (C6) is required. It is required if the ESR zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid:

F S>1

OUT ESR

If this is the case, then add the second compensation capacitor (C6) to set the pole F P3 at the location of the ESR zero. Determine the C6 value by the equation:

C6=C OUT×R ESR3

External Bootstrap Diode

It is recommended that an external bootstrap diode be added when the system has a 5V fixed input or the power supply generates a 5V output. This helps improve the

efficiency of the regulator. The bootstrap diode can be a low cost one such as IN4148

or BAT54( as shown in Figure 3).

This diode is also recommended for high duty cycle operation when output voltage (VOUT>12V) applications.

TYPICAL APPLICATION CIRCUIT

+24V

KTG 8502

PACKAGE DIMENSIONS:

SOP-8

PACKAGE DIMENSIONS:

ESOP-8

KTG 8502

2A.26V Synchronous Step-Down Converter

KTG8502