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LTC3778

Wide Operating Range, No RSENSE™ Step-Down Controller

Linear Technology 

LTC3778

PI FU CTIO S

RUN/SS (Pin 1): Run Control and Soft-Start Input. A

capacitor to ground at this pin sets the ramp time to full

output current (approximately 3s/µF) and the time delay

for overcurrent latchoff (see Applications Information).

Forcing this pin below 0.8V shuts down the device.

VON (Pin 2): On-Time Voltage Input. Voltage trip point for

the on-time comparator. Tying this pin to the output

voltage makes the on-time proportional to VOUT. The

comparator input defaults to 0.7V when the pin is grounded,

2.4V when the pin is tied to INTVCC.

PGOOD (Pin 3): Power Good Output. Open drain logic

output that is pulled to ground when the output voltage is

not within ±10% of the regulation point.

VRNG (Pin 4): Sense Voltage Range Input. The voltage at

this pin is ten times the nominal sense voltage at maximum

output current and can be set from 0.5V to 2V by a resistive

divider from INTVCC. The nominal sense voltage defaults

to 70mV when this pin is tied to ground, 140mV when tied

to INTVCC.

ITH (Pin 5): Current Control Threshold and Error Amplifier

Compensation Point. The current comparator threshold

increases with this control voltage. The voltage ranges

from 0V to 2.4V with 0.8V corresponding to zero sense

voltage (zero current).

FCB (Pin 6): Forced Continuous Input. Tie this pin to

ground to force continuous synchronous operation at low

load, to INTVCC to enable discontinuous mode operation at

low load or to a resistive divider from a secondary output

when using a secondary winding.

SGND (Pin 7): Signal Ground. All small-signal compo-

nents and compensation components should connect to

this ground, which in turn connects to PGND at one point.

ION (Pin 8): On-Time Current Input. Tie a resistor from VIN

to this pin to set the one-shot timer current and thereby set

the switching frequency.

VFB (Pin 9): Error Amplifier Feedback Input. This pin

connects the error amplifier input to an external resistive

divider from VOUT.

EXTVCC (Pin 10): External VCC Input. When EXTVCC ex-

ceeds 4.7V, an internal switch connects this pin to INTVCC

and shuts down the internal regulator so that controller

power is drawn from EXTVCC. Do not exceed 7V at this pin

and ensure that EXTVCC < VIN.

VIN (Pin 11): Main Input Supply. Decouple this pin to

SGND with an RC filter (1Ω, 0.1µF).

INTVCC (Pin 12): Internal 5V Regulator Output. The inter-

nal control circuits are powered from this voltage. De-

couple this pin to power ground with a minimum of 1µF

low ESR tantalum or ceramic capacitor.

DRVCC (Pin 13): Voltage Supply to Bottom Gate Driver.

Normally connected to the INTVCC pin through a decoupling

RC filter (1Ω, 0.1µF). Decouple this pin to power ground

with a minimum of 4.7µF low ESR tantalum or ceramic

capacitor. Do not exceed 7V at this pin.

BG (Pin 14): Bottom Gate Drive. Drives the gate of the

bottom N-channel MOSFET between ground and DRVCC.

PGND (Pin 15): Power Ground. Connect this pin closely to

the source of the bottom N-channel MOSFET or to the

bottom of the sense resistor when used, the (–) terminal

of CVCC and the (–) terminal of CIN.

SENSE– (Pin 16): Current Sense Comparator Input. The

(–) input to the current comparator is used to accurately

Kelvin sense the bottom side of the sense resistor or

MOSFET.

SENSE+ (Pin 17): Current Sense Comparator Input. The

(+) input to the current comparator is normally connected

to the SW node unless using a sense resistor (See Appli-

cations Information).

SW (Pin 18): Switch Node. The (–) terminal of the boot-

strap capacitor CB connects here. This pin swings from a

diode voltage drop below ground up to VIN.

TG (Pin 19): Top Gate Drive. Drives the top N-channel

MOSFET with a voltage swing equal to DRVCC superim-

posed on the switch node voltage SW.

BOOST (Pin 20): Boosted Floating Driver Supply. The (+)

terminal of the bootstrap capacitor CB connects here. This

pin swings from a diode voltage drop below DRVCC up to

VIN + DRVCC.

3778f

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