SC1186CSW View Datasheet(PDF) - Semtech Corporation

Part Name

Description

MFG CO.

SC1186CSW

PROGRAMMABLE SYNCHRONOUS DC/DC CONVERTER, DUAL LOW DROPOUT REGULATOR CONTROLLER

Semtech Corporation 

PRELIMINARY - December 2, 1999

PROGRAMMABLE SYNCHRONOUS DC/DC

CONVERTER, DUAL LOW DROPOUT

REGULATOR CONTROLLER

SC1186

Using 1.5X Room temp RDS(ON) to allow for temperature

rise.

FET type

IRL34025

IRL2203

RDS(on) (mΩ) PD (W)

15

1.69

10.5

1.19

Package

D2PAK

D2PAK

Si4410 20

2.26 SO-8

BOTTOM FET - Bottom FET losses are almost entirely

due to conduction. The body diode is forced into con-

duction 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 low

switching losses. Conduction losses for the FET can be

determined by:

PCOND = I2O ⋅ RDS(on) ⋅ (1− δ)

For the example above:

FET type

IRL34025

IRL2203

Si4410

RDS(on) (mΩ) PD (W)

15

1.33

10.5

0.93

20

1.77

Package

D2PAK

D2PAK

SO-8

Each of the package types has a characteristic thermal

impedance, for the TO-220 package, thermal

impedance is mostly determined by the heatsink used.

For the surface mount packages on double sided FR4, 2

oz printed circuit board material, thermal impedances of

40oC/W for the D2PAK and 80oC/W for the SO-8 are

readily achievable. The corresponding temperature rise

is detailed below:

Temperature rise (oC)

FET type Top FET Bottom FET

IRL34025 67.6

53.2

IRL2203 47.6

37.2

Si4410 180.8

141.6

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 ac-

cordingly. 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.

SHORT CIRCUIT PROTECTION - LINEARS

The Short circuit feature on the linear controllers is

implemented by using the Rds(on) of the FETs. As

output current increases, the regulation loop maintains

the output voltage by turning the FET on more and

more. Eventually, as the Rds(on) limit is reached, the

FET will be unably to turn on more fully, and output

voltage will start to fall. When the output voltage falls

to approximately 50% of nominal, the LDO controller

is latched off, setting output voltage to 0. Power must

be cycled to reset the latch.

To prevent false latching due to capacitor inrush cur-

rents or low supply rails, the current limit latch is ini-

tially disabled. It is enabled at a preset time (nominally

2mS) after both the LDOV and LDOEN rails rise

above their lockout points.

To be most effective, the linear FET Rds(on) should

not be selected artificially low, the FET should be cho-

sen so that, at maximum required current, it is almost

fully turned on

If, for example, a linear supply of 1.5V at 4A is re-

quired from a 3.3V ± 5% rail, max allowable Rds(on)

would be.

Rds(on)max = (0.95*3.3-1.5)/4 ≈ 400mΩ

To allow for temperature effects 200mΩ would be a

suitable room temperature maximum, allowing a peak

short circuit current of approximately 15A for a short

time before shutdown.

It is apparent that single SO-8 Si4410 are not adequate

for this application, but by using parallel pairs in each po-

sition, power dissipation will be approximately halved and

temperature rise reduced by a factor of 4.

© 1999 SEMTECH CORP.

11

652 MITCHELL ROAD NEWBURY PARK CA 91320

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