Datasheet

LTC3809-1
7
38091fc
PIN FUNCTIONS
MODE (Pin 1): This pin performs two functions: 1) auxiliary
winding feedback input, and 2) Burst Mode operation,
pulse skipping or forced continuous mode select.
To select Burst Mode operation at light loads, tie this
pin to V
IN
. Grounding this pin selects forced continuous
operation which allows the inductor current to reverse.
Tying this pin to V
FB
selects pulse-skipping mode. Do not
leave this pin fl oating.
TRACK/SS (Pin 2): Tracking Input for the Controller or
Optional External Soft-Start Input. This pin allows the
start-up of V
OUT
to “track” the external voltage at this pin
using an external resistor divider. Tying this pin to V
IN
allows V
OUT
to start up with the internal 0.74ms soft-start.
An external soft-start can be programmed by connecting
a capacitor between this pin and ground. Do not leave
this pin fl oating.
V
FB
(Pin 3): Feedback Pin. This pin receives the remotely
sensed feedback voltage for the controller from an external
resistor divider across the output.
I
TH
(Pin 4): Current Threshold and Error Amplifi er
Compensation Point. Nominal operating range on this pin
is from 0.7V to 2V. The voltage on this pin determines the
threshold of the main current comparator.
RUN (Pin 5): Run Control Input. Forcing this pin below
1.1V shuts down the chip. Driving this pin to V
IN
or
releasing this pin enables the chip to start-up with the
internal soft-start.
IPRG (Pin 6): Three-State Pin to Select Maximum Peak
Sense Voltage Threshold. This pin selects the maximum
allowed voltage drop between the V
IN
and SW pins
(i.e., the maximum allowed drop across the external
P-channel MOSFET). Tie to V
IN
, GND or fl oat to select
204mV, 85mV or 125mV respectively.
BG (Pin 7): Bottom (NMOS) Gate Drive Output. This pin
drives the gate of the external N-channel MOSFET. This
pin has an output swing from PGND to V
IN
.
TG (Pin 8): Top (PMOS) Gate Drive Output. This pin drives
the gate of the external P-channel MOSFET. This pin has
an output swing from PGND to V
IN
.
V
IN
(Pin 9): Chip Signal Power Supply. This pin powers
the entire chip, the gate drivers and serves as the positive
input to the differential current comparator.
SW (Pin 10): Switch Node Connection to Inductor. This
pin is also the negative input to the differential current
comparator and an input to the reverse current comparator.
Normally this pin is connected to the drain of the external
P-channel MOSFET, the drain of the external N-channel
MOSFET and the inductor.
GND (Exposed Pad, Pin 11): Ground connection for
internal circuits, the gate drivers and the negative input to
the reverse current comparator. The Exposed Pad must
be soldered to the PCB ground.