IR3838MPBF datasheet(25/35 Pages) IRF

25 / 35 page

IR3838MPbF

Rev 1.41

Select crossover frequency F

0=100 kHz

Since F

LC<F0<Fs/2<FESR, Type III is selected to

place the pole and zeros.

Detailed calculation of compensation Type III :

kΩ

Select

kΩ

2.01

kΩ

4.02

Select

kΩ

3.98

Ω

127

Select

Ω

128

;

and

Calculate

150

Select

159

Select

nF,

;

3.32

Select

kΩ

and

Calculate

2.2

Select

kHz

300

0.5

and

kHz

8.82

Select

kHz

567.1

kHz

17.63

Margin

Phase

Desired





























;

sin

ref

osc

Programming the Current-Limit

The Current-Limit threshold can be set by

connecting a resistor (R

OCSet) from the SW pin

to the OCSet pin. The resistor can be calculated

by using equation (4). This resistor (R

OCSet)must

be placed close to the IC.

The

DS(on)

has

positive

temperature

coefficient and it should be considered for the

worst case operation (40% increase due to

temperature has been considered in below).

The optional filter capacitor from OCSet pin to

PGnd has not been used for this design.

Setting the Power Good Threshold

In this design IR3838 is used in normal (non-

tracking, non-sequencing) mode, therefore the

PGood thresholds are internally set at 85% and

115% of Vref. At startup as soon as the internal

soft start signal reaches 2V (Figure 11), and

assuming Fb voltage follows Vref, the PGood is

asserted. As long as the voltage at the Fb pin is

between the thresholds (mentioned above),

Enable is high, and no fault happens, the PGood

remains high.

The PGood is an open drain output. Hence, it is

necessary to use a pull up resistor, R

PG, from

PGood pin to Vcc.

The value of the pull-up

resistor must be chosen such as to limit the

current flowing into the PGood pin to less than

5mA when the output voltage is not in regulation.

A typical value used is 10kΩ.

Vref Bypass Capacitor

A bypass capacitor of about 0.1uF is required to

be placed between Vref and Gnd pins. This

capacitor should be placed as close as possible

to Vref pin.

kΩ

6.04

Select

kΩ

kHz)

600

(at

μA

29.54

)

current

output

nominal

over

(50%

1.5

mΩ

1.4

mΩ

OCSet







)

LIM

(

SET

)

(

(33)

...

..........

)

(

OCSet

)

critical

(

SET





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