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 2007 Semtech Corp.

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POWER MANAGEMENT

SC1189

Theory of Operation (Linear OCP)

The Linear controllers in the SC1189 have built in

Overcurrent Protection (OCP). An overcurrent is assumed

to have occured when the external FET is turned fully on

and the output currrent is R

by the gate voltage going very high while the output volt-

age is below approximately 40% of it’s setpoint. To allow

for capacitor charging and very short overcurrent dura-

tions, the gate voltage is ramped very slowly upwards when-

ever the output voltage is below the OCP threshold. To

guarantee that the LDO output voltage is capable of reach-

ing it’s setpoint, the gate drive is disabled until both LDOV

Undervoltage Lockout (UVLO) and LDOEN Threshold val-

ues are exceeded, ensuring that there is sufficient gate

drive capability and sufficient LDO input voltage capabil-

ity. A block diagram of one LDO controller is shown below.

+

-

gm

+

-

LDOV

1.3V

LDOV-0.7V

10nA

LDOSx

+

-

12V

RESET BY

LDOV LOW

VREF

10pF

R

S

Q

14uA

1.26V

+

-

GATEx

R

+

-

R1

R

+

AGND

LDOEN

LDOV

Vout

3.3V

R2

SWITCH CLOSED

ON LOW

C RAMP

S1

During a normal start-up, once LDOV and LDOEN have

reached their thresholds, the GATEx pin is released and

C

ramp at 10nA/10pF = 1V/ms. Once the GATEx output has

ramped to the external FET threshold, Vout starts to ramp

up, following GATEx. When Vout reaches the OCP thresh-

old, approximately 40% of setpoint, switch S1 is closed

and GATEx ramps up at a much faster rate, followed by

Vout, until Vout reaches setpoint and the loop settles into

steady state regulation.

1V/ms

Gate

Vout

Vout/2

1.4V/us

Time

Startup with no short circuit

If at some later time, a short circuit is applied to the out-

put, the GATEx voltage will ramp up quickly as Vout falls to

try and maintain regulation. Once Vout has fallen to the

OCP threshold, switch S1 will open and the gate will con-

tinue ramping at the 1V/ms rate. If the short is not re-

moved before the GATEx output reaches approximately

LDOV - 0.7V, the GATEx pin will be latched low, disabling

the LDO

1V/ms

Gate

LDOV-0.7V

Vout/2

Vout

Short

applied

Time

Short circuit after startup

If the LDO tries to start into a short, the gate ramps at the

1V/ms rate to LDOV - 0.7V, where the GATEx pin will be

latched low.

1V/ms

Gate

LDOV-0.7V

Time

Startup into short circuit