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IR3638SPBF

HIGH FREQUENCY SYNCHRONOUS PWM BUCK CONTROLLER FOR TRACKING APPLIACTIONS

International Rectifier 

Circuit Description

THEORY OF OPEARTION

Introduction

The IR3638 is a voltage mode PWM

synchronous controller and operates with a fixed

400kHz switching frequency, allowing the use of

small external components. The output voltage is

set by feedback pin (Fb) and the external

reference voltage (Vp). These are two inputs to

error amplifier. The error signal between these

two inputs is compared to a fixed frequency

linear sawtooth ramp and generates fixed

frequency pulses of variable duty-cycle (D) which

drivers N-channel external MOSFETs.

The timing of the IC is controlled by an internal

oscillator circuit that uses on-chip capacitor to set

the switching frequency.

Under-Voltage Lockout

The under-voltage lockout circuit monitors the

Vcc and Enable input and assures that the

MOSFET driver outputs remain in the off state

whenever the Vcc or Enable voltages drop below

set thresholds.

Pre-Bias Startup

IR3638 is able to start up into pre-charged

output, which prevents oscillation and

disturbances of the output voltage.

The output starts in asynchronous fashion and

keeps the synchronous MOSFET off until the soft

start reaches about 2.0V. Figure 4 shows a

typical Pre-Bias condition at start up.

Depends on system configuration, specific

amount of output capacitors may be required to

prevent discharging the output voltage.

V

Vo

Pre-Bias Voltage

(Output Voltage before startup)

Time

Fig. 4: Pre-Bias start up

3/19/07

IR3638SPbF

Error Amplifier

The IR3638 is a voltage mode controller. The

error amplifier is of transconductance type and it

is capable of operating with Type III

compensation control scheme using low ESR

output capacitance.

Short Circuit Protection

The output is protected against the short-circuit.

The IR3638 protects the circuit for shorted output

by sensing the output voltage (through the

external resistor divider). The device shuts down

the PWM signals and latches off when the output

voltage drops below 0.4V.

Start Up / Down Considerations

The “Typical Application” on page1 shows a

standard configuration where multiple input

supplies are available.

This configuration minimizes the required extra

components compared with a single input supply

configuration. As it is shown the converter

voltage (Bus Voltage) is set to 5V while the IC

including the gate driver voltage are biased with

12V supply.

In order to maintain a proper start up, it is

required that the Bus Voltage ramps up first

followed by Vcc, Vc supply.

The system will be enabled when the Vp voltage

passes the under voltage lock out threshold.

Figure 5 shows start up / down sequencing.

5V Bus Voltage

12V (Vcc, Vc)

V_CPUCore (Vp of IR3638)

0.65V

POR of IR3638

0.60V

SS (IR3638)

Vout

Fig. 5: Start Up / Down Sequence

(Refer to Figure1)

7

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