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参数资料
| 型号： | LTC3785EUF-1#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 3 A SWITCHING REGULATOR, 650 kHz SWITCHING FREQ-MAX, PQCC24 |
| 封装： | 4 X 4 MM, LEAD FREE, PLASTIC, MO-220WGGD, QFN-24 |
| 文件页数： | 3/20页 |
| 文件大小： | 213K |
| 代理商： | LTC3785EUF-1#TRPBF |
LTC3785-1
11
37851fa
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-
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
ILIMIT level at any time, the IMAX comparator is tripped and
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
placing a diode (anode tied to VOUT)andaresistorfromVOUT
to the RUN/SS pin. The current sourced into RUN/SS will be
VOUT – 0.7 divided by the resistor value. To ignore all faults
source greater than 40μA into the RUN/SS pin (At 1.225V on
theRUN/SSpin).Sincethemaximumfaultcurrentislimited,
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 VOUTislessthan1.8V,the
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 ISVOUT and
ISSW2 pins.
+
–
+
–
+
–
+
–
+
–
+
–
gm
gm = 1/20k
ILIMIT COMP
IMAX COMP
TURN
SWITCHES
B AND D ON
V = 60k/RILSET
(15k/RILSET WHEN VOUT < 1.8V)
X10
V = 90k/RILSET
0.7V
2μA
2.2V
CSS
1/3 ILIM(OUT)
10μA MAX
ILIM(OUT)
30μA MAX
1μA
RUN/SS
VOUT
FB
R1
R2
ILIM COMP
IMAX COMP
VC
CP1
1.225V
THERMAL SD
RUN
FAULT
LOGIC
ILIMIT
SET
SAMPLED
REVERSE
CURRENT LIMIT
CCM = HIGH = 6k/RILSET
CCM = LOW = 15mV
ISVIN
VIN
VOUT
COUT
L1
TG1
SW1
SW2
A
B
D
C
ISSW2
ISSW1
CCM
ISVOUT
+
–
TG2
37851 F03
SWITCH D
OFF
1.225V
ERROR AMP
+
–
ILSET
RILSET
BG1
BG2
S
1
4
3
2
5
15
14
13
12
10
6
17
18
19
20
22
Figure 3. Block Diagram of Current Limit Fault Circuitry
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| LTC3785EUF-1#PBF | 3 A SWITCHING REGULATOR, 650 kHz SWITCHING FREQ-MAX, PQCC24 |
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