LTC3711
U
U
U
PI FU CTIO S
VID0-VID4 (Pins 23, 24, 1, 12, 13): VID Digital Inputs.
The voltage identification (VID) code sets the internal
feedbackresistordividerratiofordifferentoutputvoltages
as shown in Table 1. If unconnected, the pins are pulled
high by internal 2.5µA current sources.
VFB (Pin 10): Error Amplifier Feedback Input. This pin
connectstoboththeerroramplifierinputandtotheoutput
of the internal resistive divider. It can be used to attach
additional compensation components if desired.
VOSENSE (Pin 11): Output Voltage Sense. The output
voltage connects here to the input of the internal resistive
feedback divider.
RUN/SS (Pin 2): 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.
EXTVCC (Pin 14): 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
andgatedrivepowerisdrawnfromEXTVCC.Donotexceed
7V at this pin and ensure that EXTVCC < VIN.
V
ON (Pin 3): 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
comparatorinputdefaultsto0.7Vwhenthepinisgrounded,
2.4V when the pin is tied to INTVCC.
VIN (Pin 15): Main Input Supply. Decouple this pin to
PGND with an RC filter (1Ω, 0.1µF).
INTVCC (Pin 16): 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.
PGOOD (Pin 4): 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.
VRNG (Pin 5): 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.
BG (Pin 17): Bottom Gate Drive. Drives the gate of the
bottom N-channel MOSFET between ground and INTVCC.
PGND (Pin 18):Power Ground. Connect this pin closely to
the source of the bottom N-channel MOSFET, the (–)
terminal of CVCC and the (–) terminal of CIN.
SENSE+ (Pin 19): 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).
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
atlowloadortoaresistivedividerfromasecondaryoutput
when using a secondary winding.
SW (Pin 20): 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.
ITH (Pin 7): 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).
TG (Pin 21): Top Gate Drive. Drives the top N-channel
MOSFET with a voltage swing equal to INTVCC superim-
posed on the switch node voltage SW.
SGND (Pin 8): Signal Ground. All small-signal compo-
nents and compensation components should connect to
this ground, which in turn connects to PGND at one point.
BOOST (Pin 22): 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.
ION (Pin 9): On-Time Current Input. Tie a resistor from VIN
tothispintosettheone-shottimercurrentandtherebyset
the switching frequency.
3711f
7
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