Datasheet

11
LTC3830/LTC3830-1
3830fa
External Clock Synchronization
The LTC3830 SHDN pin doubles as an external clock input
for applications that require a synchronized clock. An
internal circuit forces the LTC3830 into external synchro-
nization mode if a negative transition at the SHDN pin is
detected. In this mode, every negative transition on the
SHDN pin resets the internal oscillator and pulls the ramp
signal low, this forces the LTC3830 internal oscillator to
lock to the external clock frequency. The LTC3830-1 does
not have this external synchronization function.
The LTC3830 internal oscillator can be externally synchro-
nized from 100kHz to 500kHz. Frequencies above 300kHz
can cause a decrease in the maximum obtainable duty
cycle as rise/fall time and propagation delay take up a
larger percentage of the switch cycle. Circuits using these
frequencies should be checked carefully in applications
where operation near dropout is important—like 3.3V to
2.5V converters. The low period of this clock signal must
not be >100µs, or else the LTC3830 enters shutdown
mode.
Figure 5 describes the operation of the external synchro-
nization function. A negative transition at the SHDN pin
forces the internal ramp signal low to restart a new PWM
cycle. Notice that with the traditional sync method, the
ramp amplitude is lowered as the external clock frequency
goes higher. The effect of this decrease in ramp amplitude
increases the open-loop gain of the controller feedback
loop. As a result, the loop crossover frequency increases
and it may cause the feedback loop to be unstable if the
phase margin is insufficient.
To overcome this problem, the LTC3830 monitors the
peak voltage of the ramp signal and adjusts the oscillator
charging current to maintain a constant ramp peak.
APPLICATIO S I FOR ATIO
WUUU
SHDN
200kHz
FREE RUNNING
RAMP SIGNAL
TRADITIONAL
SYNC METHOD
WITH EARLY
RAMP
TERMINATION
LTC3830
KEEPS RAMP
AMPLITUDE
CONSTANT
UNDER SYNC
RAMP SIGNAL
WITH EXT SYNC
RAMP AMPLITUDE
ADJUSTED
3830 F05
Figure 5. External Synchronization Operation
3830 F6
+
V
CC
PV
CC2
PV
CC1
V
IN
G1
Q1
C
OUT
V
OUT
Q2
L
O
G2
INTERNAL
CIRCUITRY
LTC3830 (16-LEAD)
3830 F7
+
V
CC
/PV
CC2
PV
CC1
V
IN
G1
Q1
C
OUT
V
OUT
Q2
L
O
G2
INTERNAL
CIRCUITRY
LTC3830 (8-LEAD)
Figure 6. 16-Lead Power Supplies
Figure 7. 8-Lead Power Supplies