Datasheet

7
LT1681
1681f
UU
U
PI FU CTIO S
voltage on the pin is reduced to 1.5V, the pin becomes high
impedance and the charging cycle repeats. The oscillator
operates at twice the switching frequency of the controller.
Oscillator frequency f
OSC
can be approximated by the
relation:
fC
R
R
OSC FSET
FSET
FSET
≅+ ++
























05 10
3
810
2
64
1
1
.• •
––
–
–
SYNC (Pin 7): Oscillator Synchronization Input Pin with
TTL-Level Compatible Input. The SYNC input signal (at the
desired synchronized operating frequency) controls both
the internal oscillator (running at twice the SYNC fre-
quency) and the output switch phase. If the synchroniza-
tion function is not desired, this pin may be shorted to
ground.
The LT1681 internal oscillator drives a toggle flip-flop that
assures ≤50% duty cycle operation during oscillator free-
run. The oscillator, therefore, runs at twice the operating
frequency of the converter. The SYNC input decoder
incorporates a frequency doubling circuit for oscillator
synchronization, resetting the internal oscillator on both
the rising and falling edges of the input signal.
The SYNC input decoder also differentiates transition
phase and forces the toggle flip-flop to phase-lock with the
SYNC input. A transition to logic high on the SYNC input
signal corresponds to the initiation of a new switching
cycle (primary switches turning on pending current con-
trol) and a transition to logic low forces a primary switch
off state. As such, the maximum operating duty cycle is
equal to the duty cycle of the SYNC signal. The SYNC input
can therefore be used to reduce the maximum duty cycle
of the converter by reducing the duty cycle of the SYNC
input.
SS (Pin 8): Soft-Start. Connect a capacitor (C
SS
) from this
pin to ground.
The output voltage of the LT1681 error amplifier corre-
sponds to the peak current sense amplifier output de-
tected before resetting the switch outputs. The soft-start
circuit forces the error amplifier output to a zero sense
current for start-up. A 10µA current is forced from this pin
onto an external capacitor. As the SS pin voltage ramps
up, so does the LT1681 internally sensed current limit.
This effectively forces the internal current limit to ramp
from zero, allowing overall converter current to slowly
increase until normal output regulation is achieved. This
function reduces output overshoot on converter start-up.
The soft-start function incorporates a 1V
BE
“dead zone”
such that a zero current condition is maintained on the V
C
pin until the SS pin rises to 1V
BE
above ground.
The SS pin voltage is reset to start-up condition during
shutdown, undervoltage lockout and overvoltage or
overcurrent events, yielding a graceful converter output
recovery from these events.
V
FB
(Pin 9): Error Amplifier Inverting Input. Typically
connected to a resistor divider from the output and com-
pensation components to the V
C
pin.
The V
FB
pin is the converter output voltage feedback node.
Input bias current of ~50nA forces the pin high in the event
of an open-feedback path condition. The error amplifier is
internally referenced to 1.25V.
Values for the V
OUT
to V
FB
feedback resistor (R
FB1
) and the
V
FB
to ground resistor (R
FB2
) can be calculated to program
converter output voltage (V
OUT
) via the following relation:
V
OUT
= 1.25 • (R
FB1
+ R
FB2
)/R
FB2
V
C
(Pin 10): Error Amplifier Output. The LT1681 error
amplifier is a low impedance output inverting gain stage.
The amplifier has ample current source capability to allow
easy integration of isolation optocouplers that require bias
currents up to 10mA. External DC loading of the V
C
pin
reduces the external current sourcing capacity of the
5V
REF
pin by the same amount as the load on the V
C
pin.
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
C
pin to
the V
FB
pin and the feedback resistor connected to the V
FB
pin. Specific integrator characteristics can be configured
to optimize transient response.