IP1837 datasheet(16/40 Pages) IRF

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iP1837

Highly Integrated 35A Single‐input Voltage,

Synchronous Buck Regulator

March 5, 2012 | V1.26

97600

POR

Vss

Vout

0.2V

0.8V

Figure 10: Theoretical operation waveforms during soft‐start

OPERATING FREQUENCY

The switching frequency can be programmed between

250kHz – 1500kHz by connecting an external resistor from

RT pin to Gnd. Table 1 tabulates the oscillator frequency

versus RT.

TABLE 1: SWITCHING FREQUENCY VS. EXTERNAL RESISTOR (RT)

Fsw (kHz)

RT (kohm)

250

88.7

300

73.2

400

54.9

450

48.7

500

44.2

600

36.5

800

27.4

1000

22.1

1500

SHUTDOWN

The iP1837 can be shutdown by pulling the Enable pin

below its 1 V threshold. This will tri‐state both, the high

side driver as well as the low side driver.

TEMPERATURE‐COMPENSATED

OVER‐CURRENT PROTECTION

The over current protection is performed by sensing

current through the RDS(on) of low side MOSFET. This

method enhances the converter’s efficiency and reduces

cost by eliminating a current sense resistor. As shown in

Figure 11, an external resistor (ROCSet) is connected

between OCSet pin and the switch node (SW) which sets

the current limit set point.

An internal current source sources current (IOCSet ) out of

the OCSet pin. The internal current source develops a

voltage across ROCSet. When the low side MOSFET is

turned on, the inductor current flows through Q2 and

results in a voltage at OCSet which is given by:

(2)

)

(

)

(

)

(on

OCSet









An over current is detected if the OCSet pin goes below

ground. Hence, at the current limit threshold, VOCset=0.

Then, for a current limit setting ILtrip, ROCSet is calculated as

follows:

OCSet

Ltrip

OCSet

(3)

)

(



Hiccup

Control

PGnd

Vin

IntVCC

ROCSet

OCSet

IOCSet

Figure 11: Connection of over‐current sensing resistor

It should be noted that the iP1837 uses a temperature

compensated overcurrent protection scheme, i.e., IOCSet

varies with temperature with the same temperature

coeffecient as RDS(on), so that ILtrip depends only on Rocset

and is independent of temperature.

The value of Rocset calculated above is realized as a

parallel combination of an internal 2.7K resistor and an

external resistor connected between the Rocset and SW

pins. Table 2 shows the selection of the external Rocset

resistor for various values of the trip load current Iotrip at

Vcc = 3.3V.

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