8
LT1681
1681f
UU
U
PI FU CTIO S
The error amplifier can also be configured as a
transimpedance amplifier for use in secondary-side con-
troller applications. (See Applications Information section
for configuration and compensation details)
SENSE (Pin 11): Current Sense Amplifier (CSA) Nonin-
verting Input. Current is monitored via a ground refer-
enced current sense resistor, typically in series with the
source of the bottom-side switch FET. Internal limit cir-
cuitry provides for a maximum peak value of 150mV across
the sense resistor during normal operation.
I
MAX
(Pin 12): Primary Current Runaway Protection. The
I
MAX
pin is used to detect primary-side switch currents
and shuts down the primary switches if a current runaway
condition is detected. The I
MAX
function is not disabled
during the current sense blanking interval. The pin is
typically connected to the primary bottom-side switch
source and monitors switch current via a ground-refer-
enced current sense resistor. If the pin voltage exceeds
360mV, LT1681 switching function is disabled in 130ns.
Exceeding the I
MAX
threshold also triggers a soft-start
reset, resulting in a graceful recovery from a current
runaway event. For single-sense resistor systems, this pin
can be shorted to SENSE for protection during the blank-
ing interval or shorted to SGND if not used.
SG (Pin 13): Synchronous Switch Output Driver. This pin
can be connected directly to the gate of the synchronous
switch if small FETs are used (C
GATE
< 5000pF), however,
the use of a gate drive buffer is recommended for peak
efficiencies.
The SG pin output is synchronized and out-of-phase with
the BG output. The control timing of the SG output causes
its transition to “lead” the primary switch path during turn-
on by 150ns.
V
CC
(Pin 14): IC Local Power Supply Input. Bypass with a
capacitor at least 10 times greater than C
5VREF
to PGND.
The LT1681 incorporates undervoltage lockout that dis-
ables switching functions if V
CC
is below 8.4V. The LT1681
supports operational V
CC
power supply voltages from 9V
to 18V (20V absolute maximum).
PWRGND (Pin 15): Output Driver Ground Reference.
Connect through low impedance trace to V
IN
decoupling
capacitor.
BG (Pin 16): Bottom-Side Primary Switch/Forward Switch
Output Driver. Pin can be connected directly to gate of
primary bottom-side and forward switches if small FETs
are used (C
GATE
total < 5000pF), however, the 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.
BLKSENS (Pin 17): Blanking Sense Input. The current
sense function (via SENSE pin) is disabled while the
BLKSENS pin is below 5V. BLKSENS is typically con-
nected to the gate of the bottom-side primary switch
MOSFET.
BSTREF (Pin 18): V
BST
Supply Reference. Typically con-
nects to source of topside external power FET switch.
TG (Pin 19): Topside (Boosted) Primary Output Driver. Pin
can be connected directly to gate of primary topside
switch if small FETs are used (C
GATE
< 5000pF), however,
the use of a gate drive buffer is recommended for peak
efficiencies.
V
BST
(Pin 20): Topside Primary Driver Bootstrapped Sup-
ply. This “boosted” supply rail is referenced to the BSTREF
pin.
Supply voltage is maintained by a bootstrap capacitor tied
from the V
BST
pin to the boosted supply reference (BSTREF)
pin. The charge on the capacitor is refreshed each switch
cycle through a Schottky diode connected from the V
CC
supply (cathode) to the V
BST
pin (anode). The bootstrap
capacitor (C
BOOST
) must be at least 100 times greater than
the total load capacitance on the TG pin. A capacitor in the
range of 0.1µF to 1µF is generally adequate for most
applications. The bootstrap diode must have a reverse-
breakdown voltage greater than the converter V
IN
. The
LT1681 supports operational V
BST
supply voltages up to
90V (absolute maximum) referenced to ground.
Undervoltage lockout disables the topside switch until
V
BST
-BSTREF > 7.0V for start-up protection of the topside
switch.
