SC4612HSTRT datasheet(12/18 Pages) SEMTECH

12 / 18 page

SC4612H

Applications Information (continued)

POWER FETS - The FETs are chosen based on several

criteria with probably the most important being

power dissipation and power handling capability.

TOP FET - The power dissipation in the top FET is a

combination of conduction losses, switching losses

and bottom FET body diode recovery losses.

a) Conduction losses are simply calculated as:

. . . . . . . . . (7)

)

(

COND

cycle

Duty

where

≈

⋅

b) Switching losses can be estimated if the switching

time is known or assumed:

. . . . . . . . . (8)

(

)

OSC

⋅

c) Body diode recovery losses are more difficult to

estimate, but to a first approximation, it is reasonable

to assume 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:

. . . . . . . . . (9)

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:

. . . . . . . . . (0)

(

)

(

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.

Low Side R

DS(ON) Current Limit

Fig3: Current Limit circuitry

. Programming resistors Ra and Rb - Not installed:

. . . . . . ()

100

−

solving for: V

PH = -00mV, the circuit will trip

at R

DS(ON) x ILOAD = 00mV

2. To increase trip voltage - install Ra.

. . . . . . . . . (2)

772

⋅

−

solving for double the current limit: V

PH = -200mV.

Ra = 768kW.

3. To decrease trip voltage - install Rb

. . . . . . . . . (3)

⋅

−

solving for half the current limit: V

PH = -50mV.

Rb = 8kW.

NOTE: Allow for tempco and R

DS(ON) variation of the

MOSFET- see the“Over Current Protection”section of

the datasheet.

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