Datasheet

LTC3785
11
3785fc
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.
R
EVERSE
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.
Figure 3. Block Diagram of Current Limit Fault Circuitry
+
–
+
–
+
–
+
–
+
–
+
–
g
m
g
m
= 1/20k
I
LIMIT
COMP
I
MAX
COMP
TURN
SWITCHES
B AND D ON
V = 60k/R
ILSET
(15k/R
ILSET
WHEN V
OUT
< 1.8V)
X10
V = 90k/R
ILSET
0.7V
2µA
2.2V
C
SS
1/3 • I
LIM(OUT)
10µA MAX
I
LIM(OUT)
30µA MAX
1µA
RUN/SS
V
OUT
FB
R1
R2
I
LIM
COMP
I
MAX
COMP
V
C
C
P1
1.225V
THERMAL SD
RUN
FAULT
LOGIC
I
LIMIT
SET
SAMPLED
SAMPLED
REVERSE
CURRENT LIMIT
CCM = HIGH = 6k/R
ILSET
CCM = LOW = 15mV
I
SVIN
V
IN
V
OUT
C
OUT
L1
TG1
SW1
SW2
A
B
D
C
I
SSW2
I
SSW1
CCM
I
SVOUT
+–
TG2
3785 F03
SWITCH D
OFF
1.225V
ERROR AMP
+
–
I
LSET
R
ILSET
BG1
BG2
S
S
1
4
3
2
5
15
14
13
12
10
6
17
18
19
20
22