LTC3805-5_15 Datasheet, PDF (7/20 Pages) Linear Technology – Adjustable Frequency Current Mode Flyback/ Boost/SEPIC DC/DC Controller

LTC3805-5
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.
BLOCK DIAGRAM
9
V_CC
4
RUN
SSFLT
1
OC
8
800mV
REFERENCE
SOFT-START RAMP
10µA
OVERCURRENT
COMPARATOR
+
SOFT-START
FAULT
UNDERVOLTAGE
LOCKOUT
SHUTDOWN
FB
3
GND
11
100mV
–
ERROR
+
AMPLIFIER
–
SHUTDOWN
I_TH CLAMPS
1.2V
2 ^ITH
–
CURRENT
COMPARATOR
R
+
Q
S
SWITCHING
LOGIC AND
BLANKING
CIRCUIT
20mV
OSCILLATOR
SLOPE
COMP
CURRENT
RAMP
FS
SYNC
5
6
GATE
DRIVER
GATE
10
ISENSE
7
38055 BD
38055fe
7