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LTC1778EGN-1

Wide Operating Range, No RSENSE™ Step-Down Controller

Linear Technology 

LTC1778/LTC1778-1

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.

PGOOD (Pin 2, LTC1778): Power Good Output. Open

drain logic output that is pulled to ground when the output

voltage is not within ±7.5% of the regulation point.

VON (Pin 2, LTC1778-1): On-Time Voltage Input. Voltage

trip point for the on-time comparator. Tying this pin to the

output voltage or an external resistive divider from the

output makes the on-time proportional to VOUT. The

comparator input defaults to 0.7V when the pin is grounded

or unavailable (LTC1778) and defaults to 2.4V when the

pin is tied to INTVCC. Tie this pin to INTVCC in high VOUT

applications to use a lower RON value.

VRNG (Pin 3): Sense Voltage Range Input. The voltage at

this pin is ten times the nominal sense voltage at maxi-

mum 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.

FCB (Pin 4): 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.

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).

SGND (Pin 6): 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 7): 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 8): Error Amplifier Feedback Input. This pin

connects the error amplifier input to an external resistive

divider from VOUT.

EXTVCC (Pin 9): 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

and gate drive power is drawn from EXTVCC. Do not exceed

7V at this pin and ensure that EXTVCC < VIN.

VIN (Pin 10): Main Input Supply. Decouple this pin to

PGND with an RC filter (1Ω, 0.1µF).

INTVCC (Pin 11): Internal 5V Regulator Output. The driver

and control circuits are powered from this voltage. De-

couple this pin to power ground with a minimum of 4.7µF

low ESR tantalum capacitor.

BG (Pin 12): Bottom Gate Drive. Drives the gate of the

bottom N-channel MOSFET between ground and INTVCC.

PGND (Pin 13): Power Ground. Connect this pin closely to

the source of the bottom N-channel MOSFET, the (–)

terminal of CVCC and the (–) terminal of CIN.

SW (Pin 14): 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 15): Top Gate Drive. Drives the top N-channel

MOSFET with a voltage swing equal to INTVCC superim-

posed on the switch node voltage SW.

BOOST (Pin 16): Boosted Floating Driver Supply. The (+)

terminal of the bootstrap capacitor CB connects here. This

pin swings from a diode voltage drop below INTVCC up to

VIN + INTVCC.

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