LTC3785_15 Datasheet, PDF (11/20 Pages) Linear Technology – 10V, High Efficiency, Synchronous, No RSENSE Buck-Boost Controller

LTC3785
operation
To have the power converter latch off on a fault, a pull-up
current between 4µA and 7µA on the RUN/SS pin will allow
the RUN/SS capacitor to discharge during an extended
fault, but will prevent cycling of the fault which will cause the
converter to stay off. One method to implement this is by
placing a diode (anode tied to V_OUT) and a resistor from V_OUT
to the RUN/SS pin. The current sourced into RUN/SS will be
V_OUT – 0.7 divided by the resistor value. To ignore all faults
source greater than 40µA into the RUN/SS pin (At 1.225V on
the RUN/SS pin). Since the maximum fault current is limited,
this will prevent any discharging of the RUN/SS capacitor,
the soft-start capacitor will need to be sized accordingly to
accommodate the extra charging current at start-up.
During an output short-circuit or if V_OUT is less than 1.8V,
the current limit folds back to 50% of the programmed
level.
Reverse Current Limit
The LTC3785 can be programmed to provide full class D
operation or allowed to source and sink current equal to
the current limit set value. This is achieved by asserting a
high level on the CCM pin. To minimize the reverse output
current, the CCM pin should be driven low or strapped to
ground. During this mode only, –15mV typical is allowed
across output switch D and is sensed with the I_SVOUT and
I_SSW2 pins.
THERMAL SD
1.225V +
–
–
0.7V
+
S FAULT
S LOGIC
RUN
g_m = 1/20k
I_LIMIT COMP
g_m
V = 60k/R_ILSET
(15k/R_ILSET WHEN V_OUT < 1.8V)
I_SVIN
22
TG1 20
RUN/SS
1
C_SS
2.2V
4
V_OUT
R1
FB
C_P1 ³
V_C
2
R2
1µA
2µA
1/3 • I_LIM(OUT)
10µA MAX
I_LIM(OUT)
30µA MAX
1.225V
+ ^{ERROR AMP}
–
I_LSET
5
R_ILSET
I_LIMIT
SET
I_LIM COMP
I_MAX COMP
I_MAX COMP
TURN
SWITCHES
B AND D ON
V = 90k/R_ILSET
SW1 19
X10
I_SSW1
SAMPLED
18
BG1 17
CCM = HIGH = 6k/R_ILSET
CCM = LOW = 15mV
CCM
6
SWITCH D
OFF
REVERSE
CURRENT LIMIT
–+
I_SVOUT
10
TG2 12
SW2 13
SAMPLED ^ISSW2 14
BG2 15
V_IN
A
B
L1
V_OUT
D
C_OUT
C
3785 F03
Figure 3. Block Diagram of Current Limit Fault Circuitry
3785fc
11