LTC3830-1 Linear Technology, LTC3830-1 Datasheet - Page 11

• Manufacturer Part Number: LTC3830-1
• Description: High Power Step-Down Synchronous DC/DC Controllers for Low Voltage Operation
• Manufacturer: Linear Technology

SYNC METHOD
APPLICATIO S I FOR ATIO
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 >100 s, 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.
TERMINATION
UNDER SYNC
TRADITIONAL
KEEPS RAMP
WITH EARLY
AMPLITUDE
CONSTANT
LTC3830
RAMP
RAMP AMPLITUDE
Figure 5. External Synchronization Operation
SHDN
FREE RUNNING
ADJUSTED
RAMP SIGNAL
200kHz
U
U
W
WITH EXT SYNC
RAMP SIGNAL
U
3830 F05
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.
Input Supply Considerations/Charge Pump
The 16-lead LTC3830 requires four supply voltages to
operate: V
MOSFET gate drive and a clean, low ripple V
LTC3830 internal circuitry (Figure 6). The LTC3830-1 and
the 8-lead LTC3830 have the PV
together inside the package (Figure 7). This pin, brought
out as V
as the 16-lead part, but must also be able to supply the gate
drive current to Q2.
In many applications, V
through an RC filter. This supply can be as low as 3V. The
low quiescent current (typically 800 A) allows the use of
relatively large filter resistors and correspondingly small
LTC3830 (16-LEAD)
LTC3830 (8-LEAD)
CIRCUITRY
CIRCUITRY
INTERNAL
V
INTERNAL
CC
V
CC
/PV
CC
IN
CC2
/PV
for the main power input, PV
Figure 6. 16-Lead Power Supplies
CC2
Figure 7. 8-Lead Power Supplies
PV
, has the same low ripple requirements
CC2
LTC3830/LTC3830-1
PV
PV
CC
CC1
CC1
can be powered from V
G1
G2
G1
G2
CC2
V
V
IN
IN
Q1
Q2
Q1
Q2
and V
CC1
L
L
O
O
and PV
CC
+
+
CC
pins tied
sn3830 3830fs
3830 F6
3830 F7
11
for the
C
C
CC2
OUT
OUT
V
V
OUT
OUT
for
IN