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SC1186 View Datasheet(PDF) - Semtech Corporation

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SC1186 Datasheet PDF : 15 Pages
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SC1186
POWER MANAGEMENT
Theory of Operation (Linear OCP)
The Linear controllers in the SC1186 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 RDS(ON) limited, this is detected
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.
12V
LDOV
LDOEN
3.3V
VREF
1.26V
+
-
-
gm
+
10pF
C RAMP
LDOV
-
+
1.3V R2
GATEx
LDOSx
Vout
1.4V/us
1V/ms
Gate
Vout
Vout/2
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
Short
applied
1V/ms
Gate
LDOV-0.7V
SWITCH CLOSED
R
ON LOW
-
S1
+
10nA
R
R1
14uA
RESET BY
LDOV LOW
S
Q
R
+
-
LDOV-0.7V
+
AGND
Vout
Vout/2
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.
During a normal start-up, once LDOV and LDOEN have
reached their thresholds, the GATEx pin is released and
CRAMP is charged by 10nA causing the GATEx voltage to
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.
2004 Semtech Corp.
11
1V/ms
Gate
LDOV-0.7V
Time
Startup into short circuit
www.semtech.com
 

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