Datasheet

LTC3805-5
7
38055fe
BLOCK DIAGRAM
PIN FUNCTIONS
SSFLT (Pin 1): Soft-Start Pin. A capacitor placed from
this pin to GND (Exposed Pad) controls the rate of rise of
converter output voltage during start-up. This capacitor is
also used for time out after a fault prior to restart.
I
TH
(Pin 2): Error Amplifier Compensation Point. Normal
operating voltage range is clamped between 0.7V and
1.9V.
FB (Pin 3): Receives the feedback voltage from an external
resistor divider across the output.
RUN (Pin 4): An external resistor divider connects this pin
to V
IN
and sets the thresholds for converter operation.
FS (Pin 5): A resistor connected from this pin to ground
sets the frequency of operation.
SYNC (Pin 6): Input to synchronize the oscillator to an
external source.
I
SENSE
(Pin 7): Performs two functions: for current mode
control, it monitors the switch current, using the voltage
across an external current sense resistor. Pin 7 also injects
a current ramp that develops slope compensation voltage
across an optional external programming resistor.
OC (Pin 8): Overcurrent Pin. Connect this pin to the ex-
ternal switch current sense resistor. An additional resistor
programs the overcurrent trip level.
V
CC
(Pin 9): Supply Pin. A capacitor must closely decouple
V
CC
to GND (Exposed Pad).
GATE (Pin 10): Gate Drive for the External N-Channel
MOSFET. This pin swings from GND to V
CC
.
GND (Exposed Pad Pin 11): Ground. A capacitor must
closely decouple GND to V
CC
(Pin 9). The exposed pad
must be soldered to electrical ground on PCB for electrical
contact and rated thermal performance.
–
+
–
+
–
+
SLOPE
COMP
CURRENT
RAMP
V
CC
RUN
GATE
DRIVER
GATE
7
ISENSE
38055 BD
OSCILLATOR
Q
R
CURRENT
COMPARATOR
OVERCURRENT
COMPARATOR
SHUTDOWN
SHUTDOWN
I
TH
CLAMPS
S
20mV
I
TH
ERROR
AMPLIFIER
FB
SOFT-START RAMP
10µA
100mV
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
10
3
SSFLT
1
OC
8
GND
1.2V
11
2
SYNCFS
5 6
9 4
UNDERVOLTAGE
LOCKOUT
800mV
REFERENCE
SOFT-START
FAULT