sc1189 Semtech Corporation, sc1189 Datasheet - Page 12

• Manufacturer Part Number: sc1189
• Description: Sc1189 Programmable Synchronous Dc/dc Converter, Dual Ldo Controller
• Manufacturer: Semtech Corporation

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.
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.
POWER MANAGEMENT
Theory of Operation (Linear OCP)
RAMP
2007 Semtech Corp.
RESET BY
LDOV LOW
VREF
1.26V
is charged by 10nA causing the GATEx voltage to
R
R
12V
LDOV
Q
R
R1
S
+
-
SWITCH CLOSED
ON LOW
LDOEN
-
+
gm
+
-
14uA
S1
C RAMP
1.3V
DS(ON)
10pF
10nA
LDOV-0.7V
+
-
-
+
R2
LDOV
limited, this is detected
GATEx
LDOSx
AGND
3.3V
+
Vout
12
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
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.
applied
Short
1V/ms
1V/ms
Startup with no short circuit
Short circuit after startup
Vout
Startup into short circuit
Vout/2
1V/ms
1.4V/us
Gate
Gate
Time
Time
Time
Vout/2
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SC1189
LDOV-0.7V
LDOV-0.7V
Gate
Vout