LTC3785-1 Linear Technology, LTC3785-1 Datasheet - Page 11

• Manufacturer Part Number: LTC3785-1
• Description: Buck-Boost Controller
• Manufacturer: Linear Technology

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OPERATION
of the voltage loop, resulting in a lower output voltage
to regulate the input current. This fault condition causes
the RUN/SS capacitor to begin discharging. The level of
the discharge current depends on how much the current
exceeds the programmed threshold. Figure 3 is a simpli-
fi ed diagram of the current sense and fault circuitry. If the
current limit fault duration is long enough to discharge the
RUN/SS capacitor below 1.225V, the fault latch is set and
will cycle the RUN/SS capacitor 16 times (1μA charging
and 1μA discharging of the RUN/SS capacitor) to create an
off time of 32 times the soft-start time before the outputs
are allowed to switch to restart the output voltage. If the
current limit fault level exceeds 150% of the programmed
I
output switches B and D are turned on to discharge the
inductor current for the remainder of the cycle.
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
LIMIT
V
OUT
level at any time, the I
R
R1
R2
C
ILSET
P1
C
SS
1
4
3
2
5
RUN/SS
FB
V
I
LSET
C
THERMAL SD
1.225V
0.7V
2.2V
I
LIMIT
SET
1.225V
1μA
+
–
–
+
MAX
2μA
30μA MAX
I
I
I
LIM(OUT)
LIM
MAX
comparator is tripped and
+
–
RUN
COMP
S
S
COMP
Figure 3. Block Diagram of Current Limit Fault Circuitry
ERROR AMP
LOGIC
1/3 • I
10μA MAX
FAULT
LIM(OUT)
(15k/R
B AND D ON
SWITCHES
ILSET
V = 60k/R
g
TURN
m
WHEN V
= 1/20k
V = 90k/R
ILSET
OUT
I
< 1.8V)
I
LIMIT
MAX
SWITCH D
placing a diode (anode tied to V
to the RUN/SS pin. The current sourced into RUN/SS will be
V
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
current limit folds back to 50% of the programmed level.
REVERSE CURRENT LIMIT
The LTC3785-1 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
I
g
SSW2
m
ILSET
+
–
+
–
OUT
COMP
COMP
OFF
CURRENT LIMIT
– 0.7 divided by the resistor value. To ignore all faults
pins.
REVERSE
CCM = HIGH = 6k/R
X10
+
–
+
–
CCM = LOW = 15mV
–
SAMPLED
SAMPLED
+
ILSET
I
I
SVOUT
I
SSW1
SSW2
I
SW1
CCM
SW2
OUT
SVIN
TG1
BG1
TG2
BG2
) and a resistor from V
22
20
19
18
17
10
12
13
14
15
6
OUT
LTC3785-1
is less than 1.8V, the
V
IN
A
B
D
C
L1
37851 F03
SVOUT
C
V
OUT
11
OUT
37851f
and
OUT