SC4612HSTRT datasheet(14/21 Pages) SEMTECH

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SC4612H

POWER MANAGEMENT

Low Side R

DS_ON Current Limit

1. Programming resistors Ra and Rb - Not installed:

Vphase

100

−

solving for: V

PHASE = -100mV, therefore the circuit will trip @

DS_ON x ILOAD = 100mV

2. To increase trip voltage - install Ra.

PHASE

772

⋅

−

solving for double the current limit: V

PHASE = -200mV.

Ra = 768kΩ.

3. To decrease trip voltage - install Rb

PHASE

⋅

−

solving for half the current limit: V

PHASE = -50mV.

Rb = 18kΩ.

NOTE! Allow for tempco and R

DS_ON variation of the MOS-

FET - see “overcurrent protection” information on page 11

in the datasheet.

that the stored charge on the bottom FET body diode will

be moved through the top FET as it starts to turn on. The

resulting power dissipation in the top FET will be:

OSC

⋅

BOTTOM FET - Bottom FET losses are almost entirely due

to conduction. The body diode is forced into conduction at

the beginning and end of the bottom switch conduction

period, so when the FET turns on and off, there is very little

voltage across it resulting in very low switching losses.

Conduction losses for the FET can be determined by:

)

(

)

(

COND

−

⋅

INPUT CAPACITORS - Since the RMS ripple current in the

input capacitors may be as high as 50% of the output

current, suitable capacitors must be chosen accordingly.

Also, during fast load transients, there may be restrictions

on input di/dt. These restrictions require useable energy

storage within the converter circuitry, either as extra output

capacitance or, more usually, additional input capacitors.

Choosing low ESR input capacitors will help maximize ripple

rating for a given size.

Application Information (Cont.)

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