lt3781 Linear Technology Corporation, lt3781 Datasheet - Page 8

• Manufacturer Part Number: lt3781
• Description: Bootstrap Start Dual Transistor Synchronous Forward Controller
• Manufacturer: Linear Technology Corporation

PI FU CTIO S
LT3781
The error amplifier is typically configured using a feedback
RC network to realize an integrator circuit. This circuit
creates the dominant pole for the converter regulation
feedback loop. Integrator characteristics are dominated
by the value of the capacitor connected from the V
the V
pin. Specific integrator characteristics can be configured
to optimize transient response.
The error amplifier can also be configured as a
transimpedance amplifier for use in secondary-side con-
troller applications. (See the Applications Information
section for configuration and compensation details)
SENSE (Pin 11): Current Sense Amplifier (CSA)
Noninverting Input. Current is monitored via a ground
referenced current sense resistor, typically in series with
the source of the bottom side switch FET. Internal current
limit circuitry provides for a maximum peak value of
150mV across the sense resistor during normal opera-
tion.
SG (Pin 12): Synchronous Switch Output Driver. This pin
can be connected directly to gate of synchronous switch
if small FETs are used (C
of a gate drive buffer is recommended for peak efficien-
cies.
The SG pin output is synchronized and out-of-phase with
the BG output. The control timing of the SG output cause
it to “lead” the primary switch path during turn-on by
150nS.
V
a capacitor at least 10 times greater than C5V
incorporates undervoltage lockout that disables switching
functions if V
operational V
(20V absolute maximum). An 18V clamp on the V
enabled during shutdown mode, preventing a trickle start
circuit from pulling that pin above maximum operational
levels during IC shutdown.
PWRGND (Pin 14): Output Driver Ground Reference.
Connect through low impedance trace to V
capacitor.
8
CC
U
(Pin 13): IC Local Power Supply Input. Bypass with at
FB
pin and the feedback resistor connected to the V
U
CC
CC
power supply voltages from 9V to 18V
is below 8.4V. The LT3781 supports
U
GATE
< 5000pF), however, the use
IN
REF
decoupling
. LT3781
CC
C
pin to
pin is
FB
BG (Pin 15): Bottom Side Primary Switch/Forward Switch
Output Driver. This pin can be connected directly to
gate(s) of primary bottom side and forward switches if
small FETs are used (C
use of a gate drive buffer is recommended for peak
efficiencies.
The BG output is enabled at the start of each oscillator
cycle in phase with the TG pin but is timed to “lag” the TG
output during turn-on and “lead” the TG output during
turn-off. These delays force the concentration of transi-
tional losses onto the bottom side primary switch.
An adaptive blanking circuit disables the current sense
function (via the SENSE pin) while the BG pin is below 5V.
BSTREF (Pin 18): V
nects to source of topside external power FET switch.
TG (Pin 19): Topside (Boosted) Primary Output Driver.
This pin can be connected directly to gate of primary
topside switch if small FETs are used (C
however, the use of a gate drive buffer is recommended for
peak efficiencies.
V
ply. This “boosted” supply rail is referenced to the BSTREF
pin.
Supply voltage is maintained by a bootstrap capacitor tied
from the V
pin. The charge on the capacitor is refreshed each switch
cycle through a Schottky diode connected from the V
supply (cathode) to the V
capacitor (C
the total load capacitance on the TG pin. A capacitor in the
range of 0.1 F to 1.0 F is generally adequate for most
applications. The bootstrap diode must have a reverse
breakdown voltage greater than the converter V
LT3781 supports operational V
90V (absolute maximum) referenced to ground.
Undervoltage Lockout disables the topside switch until
V
topside switch.
BST
BST
(Pin 20): Topside Primary Driver Bootstrapped Sup-
– BSTREF > 7.0V for start-up protection of the
BST
BOOST
pin to the boosted supply reference (BSTREF)
) must be at least 100 times greater than
BST
GATE
Supply Reference. Typically con-
BST
total < 5000pF), however, the
pin (anode). The bootstrap
BST
supply voltages up to
GATE
< 5000pF),
IN
. The
3781f
CC