IR3637ASPBF datasheet(5/19 Pages) IRF

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IR3637ASPbF

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THEORY OF OPERATION

Soft-Start

The IR3637A has a programmable soft-start to control

the output voltage rise and limit the current surge at the

start-up. To ensure correct start-up, the soft-start se-

quence initiates when the Vc and Vcc rise above their

threshold (3.3V and 4.2V respectively) and generates

the Power On Reset (POR) signal. Soft-start function

operates by sourcing an internal current to charge an

external capacitor to about 3V. Initially, the soft-start func-

tion clamps the E/A’s output of the PWM converter and

disables the short circuit protection. During the power

up, the output starts at zero and voltage at Fb is below

0.4V. The feedback UVLO is disabled during this time

by injecting a current (64µA) into the Fb. This generates

a voltage about 1.6V (64µA×25K) across the negative

input of E/A and positive input of the feedback UVLO

comparator (see Figure 3).

The magnitude of this current is inversely proportional to

the voltage at soft-start pin.

The 20µA current source starts to charge up the exter-

nal capacitor. In the mean time, the soft-start voltage

ramps up, the current flowing into Fb pin starts to de-

crease linearly and so does the voltage at the positive

pin of feedback UVLO comparator and the voltage nega-

tive input of E/A.

When the soft-start capacitor is around 1V, the current

flowing into the Fb pin is approximately 32µA. The volt-

age at the positive input of the E/A is approximately:

32µA×25K = 0.8V

The E/A will start to operate and the output voltage starts

to increase. As the soft-start capacitor voltage contin-

ues to go up, the current flowing into the Fb pin will keep

decreasing. Because the voltage at pin of E/A is regu-

lated to reference voltage 0.8V, the voltage at the Fb is:

The feedback voltage increases linearly as the injecting

current goes down. The injecting current drops to zero

when soft-start voltage is around 2V and the output volt-

age goes into steady state.

As shown in Figure 4, the positive pin of feedback UVLO

comparator is always higher than 0.4V, therefore, feed-

back UVLO is not functional during soft-start.

25uA

64uA

Max

POR

Error Amp

64uA×25K=1.6V

When SS=0

POR

0.4V

Feeback

UVLO Comp

SS/SD

Comp

25K

0.8V

25K

HDrv

LDrv

VFB = 0.8-25K×(Injected Current)

The output start-up time is the time period when soft-

start capacitor voltage increases from 1V to 2V. The start-

up time will be dependent on the size of the external

soft-start capacitor. The start-up time can be estimated

by:

For a given start up time, the soft-start capacitor can be

estimated as:

Soft-Start

Voltage

Voltage at negative input

of Error Amp and Feedback

UVLO comparator

Voltage at Fb pin

Current flowing

into Fb pin

64uA

0uA

0.8V

≅1.6V

0.8V

≅2V

≅1V

Output of UVLO

POR

25µA×TSTART/CSS= 2V-1V

CSS

≅ 25µA×TSTART/1V

Figure 3 - Soft-start circuit for IR3637A.

Figure 4 - Theoretical operational waveforms

during soft-start.

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