LTC3703_15 Datasheet, PDF (7/34 Pages) Linear Technology – 100V Synchronous Switching Regulator Controller

LTC3703
Pin Functions (GN16/G28)
MODE/SYNC (Pin 1/Pin 6): Pulse-Skip Mode Enable/Sync
Pin. This multifunction pin provides pulse-skip mode
enable/disable control and an external clock input for
synchronization of the internal oscillator. Pulling this pin
below 0.8V or to an external logic-level synchronization
signal disables pulse-skip mode operation and forces
continuous operation. Pulling the pin above 0.8V enables
pulse-skip mode operation. This pin can also be connected
to a feedback resistor divider from a secondary winding
on the inductor to regulate a second output voltage.
f_SET (Pin 2/Pin 7): Frequency Set. A resistor connected
to this pin sets the free running frequency of the internal
oscillator. See Applications Information section for resistor
value selection details.
COMP (Pin 3/Pin 8): Loop Compensation. This pin is con-
nected directly to the output of the internal error amplifier.
An RC network is used at the COMP pin to compensate
the feedback loop for optimal transient response.
FB (Pin 4/Pin 9): Feedback Input. Connect FB through a
resistor divider network to V_OUT to set the output volt-
age. Also connect the loop compensation network from
COMP to FB.
I_MAX (Pin 5/Pin 10): Current Limit Set. The I_MAX pin sets
the current limit comparator threshold. If the voltage drop
across the bottom MOSFET exceeds the magnitude of the
voltage at I_MAX, the controller goes into current limit. The
I_MAX pin has an internal 12µA current source, allowing the
current threshold to be set with a single external resistor
to ground. See the Current Limit Programming section
for more information on choosing R_IMAX.
INV (Pin 6/Pin 11): Top/Bottom Gate Invert. Pulling this
pin above 2V sets the controller to operate in step-up
(boost) mode with the TG output driving the synchronous
MOSFET and the BG output driving the main switch. Below
1V, the controller will operate in step-down (buck) mode.
RUN/SS (Pin 7/Pin 13): Run/Soft-Start. Pulling RUN/SS
below 0.9V will shut down the LTC3703, turn off both of
the external MOSFET switches and reduce the quiescent
supply current to 50µA. A capacitor from RUN/SS to
ground will control the turn-on time and rate of rise of
the output voltage at power-up. An internal 4µA current
source pull-up at the RUN/SS pin sets the turn-on time
at approximately 750ms/µF.
GND (Pin 8/Pin 14): Ground Pin.
BGRTN (Pin 9/Pin 15): Bottom Gate Return. This pin
connects to the source of the pull-down MOSFET in the
BG driver and is normally connected to ground. Connect-
ing a negative supply to this pin allows the synchronous
MOSFET’s gate to be pulled below ground to help prevent
false turn-on during high dV/dt transitions on the SW node.
See the Applications Information section for more details.
BG (Pin 10/Pin 19): Bottom Gate Drive. The BG pin drives
the gate of the bottom N-channel synchronous switch
MOSFET. This pin swings from BGRTN to DRV_CC.
DRV_CC (Pin 11/Pin 20): Driver Power Supply Pin. DRV_CC
provides power to the BG output driver. This pin should
be connected to a voltage high enough to fully turn on
the external MOSFETs, normally 10V to 15V for standard
threshold MOSFETs. DRV_CC should be bypassed to BGRTN
with a 10µF, low ESR (X5R or better) ceramic capacitor.
V_CC (Pin 12/Pin 21): Main Supply Pin. All internal circuits
except the output drivers are powered from this pin. V_CC
should be connected to a low noise power supply voltage
between 9V and 15V and should be bypassed to GND
(Pin 8) with at least a 0.1µF capacitor in close proximity
to the LTC3703.
SW (Pin 13/Pin 26): Switch Node Connection to Inductor
and Bootstrap Capacitor. Voltage swing at this pin is from a
Schottky diode (external) voltage drop below ground to V_IN.
TG (Pin 14/Pin 27): Top Gate Drive. The TG pin drives the
gate of the top N-channel synchronous switch MOSFET.
The TG driver draws power from the BOOST pin and
returns to the SW pin, providing true floating drive to the
top MOSFET.
BOOST (Pin 15/Pin 28): Top Gate Driver Supply. The
BOOST pin supplies power to the floating TG driver. The
BOOST pin should be bypassed to SW with a low ESR
(X5R or better) 0.1µF ceramic capacitor. An additional fast
recovery Schottky diode from DRV_CC to BOOST will create
a complete floating charge-pumped supply at BOOST.
V_IN (Pin 16/Pin 1): Input Voltage Sense Pin. This pin is
connected to the high voltage input of the regulator and is
used by the internal feedforward compensation circuitry
to improve line regulation. This is not a supply pin.
3703fc
7