LTC3703 datasheet(7/32 Pages) LINER

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LTC3703

3703f

MODE/SYNC (Pin 1): Pulse Skip Mode Enable/Sync Pin.

This multifunction pin provides Pulse Skip Mode enable/

disable control and an external clock input for synchroni-

zation of the internal oscillator. Pulling this pin below 0.8V

or to an external logic-level synchronization signal dis-

ables 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.

fSET (Pin 2): Frequency Set. A resistor connected to this

pin sets the free running frequency of the internal oscilla-

tor. See applications section for resistor value selection

details.

COMP (Pin 3): Loop Compensation. This pin is connected

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): Feedback Input. Connect FB through a resistor

divider network to VOUT to set the output voltage. Also

connect the loop compensation network from COMP to

FB.

IMAX (Pin 5): Current Limit Set. The IMAX pin sets the

current limit comparator threshold. If the voltage drop

across the bottom MOSFET exceeds the magnitude of the

voltage at IMAX, the controller goes into current limit. The

IMAX 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 RIMAX.

INV (Pin 6): 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): 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 sup-

ply current to 50

µA. 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 4

µA current source pull-

up at the RUN/SS pin sets the turn-on time at approxi-

mately 750ms/

µF.

GND (Pin 8): Ground Pin.

BGRTN (Pin 9): Bottom Gate Return. This pin connects to

the source of the pull-down MOSFET in the BG driver and

is normally connected to ground. Connecting 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): Bottom Gate Drive. The BG pin drives the gate

of the bottom N-channel synchronous switch MOSFET.

This pin swings from BGRTN to DRVCC.

DRVCC (Pin 11): Driver Power Supply Pin. DRVCC pro-

vides 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. DRVCC shouldbebypassedtoBGRTN

with a 10

µF, low ESR (X5R or better) ceramic capacitor.

VCC (Pin 12) : Main Supply Pin. All internal circuits except

the output drivers are powered from this pin. VCC 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 0.1

µFcapacitorincloseproximitytotheLTC3703.

SW (Pin 13): Switch Node Connection to Inductor and

Bootstrap Capacitor. Voltage swing at this pin is from a

Schottky diode (external) voltage drop below ground to

VIN.

TG (Pin 14): 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): 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)

0.1

µF ceramic capacitor. An additional fast recovery

Schottky diode from DRVCC to BOOST will create a com-

plete floating charge-pumped supply at BOOST.

VIN (Pin 16): Input Voltage Sense Pin. This pin is con-

nected 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.

PI FU CTIO S (GN16)

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