LTC3836 LINER [Linear Technology], LTC3836 Datasheet - Page 11

• Manufacturer Part Number: LTC3836
• Description: Dual 2-Phase, No RSENSETM Low VIN Synchronous Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
When the SYNC/FCB pin is tied to a DC voltage above
0.6V or when it is clocked by an external clock source
to use the phase-locked loop (see Frequency Selection
and Phase-Locked Loop), the LTC3836 operates in PWM
pulse-skipping mode at light loads. In this mode, the
current comparator I
cycles and force the main N-channel MOSFET to stay off
for the same number of cycles. The inductor current is
not allowed to reverse, though (discontinuous operation).
This mode, like forced continuous operation, exhibits low
output ripple as well as low audio noise and reduced RF
interference. However, it provides low current effi ciency
higher than forced continuous mode. During start-up or
a short-circuit condition (V
LTC3836 operates in pulse-skipping mode (no cur-
rent reversal allowed), regardless of the state of the
SYNC/FCB pin.
Short-Circuit Protection
When an output is shorted to ground (V
switching frequency of that controller is reduced to one-
fi fth of the normal operating frequency. The other controller
maintains regulation in pulse-skipping mode.
The short-circuit threshold on V
of 0.12V and a fraction of the voltage on the TRACK/SS2
pin. This also allows V
more easily. Note that if V
(V
V
the TRACK/SS pin.
Output Overvoltage Protection
As a further protection, the overvoltage comparator (OV)
guards against transient overshoots, as well as other more
serious conditions that may overvoltage the output. When
the feedback voltage on the V
the reference voltage of 0.6V, the main N-channel MOSFET
is turned off and the synchronous N-channel MOSFET is
turned on until the overvoltage is cleared.
OUT2
OUT1
to 0V if a resistor divider on V
= V
FB1
= 0V), then the LTC3836 will try to regulate
CMP
(Refer to Functional Diagram)
OUT2
may remain tripped for several
FB
OUT1
to start up and track V
FB1
pin has risen 13.33% above
FB2
or V
is truly short-circuited
is based on the smaller
OUT1
FB2
FB
is connected to
≤ 0.54V), the
< 0.12V), the
OUT1
Frequency Selection and Phase-Locked Loop (PLLLPF
and SYNC/FCB Pins)
The selection of switching frequency is a tradeoff between
effi ciency and component size. Low frequency opera-
tion increases effi ciency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to maintain low output ripple voltage.
The switching frequency of the LTC3836’s controllers can
be selected using the PLLLPF pin.
If the SYNC/FCB is not being driven by an external clock
source, the PLLLPF can be fl oated, tied to V
SGND to select 550kHz, 750kHz or 300kHz respectively.
A phase-locked loop (PLL) is available on the LTC3836
to synchronize the internal oscillator to an external clock
source that is connected to the SYNC/FCB pin. In this case,
a series RC should be connected between the PLLLPF pin
and SGND to serve as the PLL’s loop fi lter. The LTC3836
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of controller 1’s top MOSFET to the ris-
ing edge of the synchronizing signal. Thus, the turn-on
of controller 2’s top MOSFET is 180 degrees out-of-phase
with the rising edge of the external clock source.
The typical capture range of the LTC3836’s phase-locked
loop is from approximately 200kHz to 1MHz, and is guar-
anteed over temperature between 250kHz and 850kHz.
In other words, the LTC3836’s PLL is guaranteed to lock
to an external clock source whose frequency is between
250kHz and 850kHz.
Dropout Operation
Each top MOSFET driver is biased from the fl oating boot-
strap capacitor C
off cycle through an external diode when the top MOSFET
turns off. If the input voltage V
close to V
turn on the top MOSFET continuously. The dropout detec-
tor detects this and forces the top MOSFET off for about
200ns every fourth cycle to allow C
OUT
, the loop may enter dropout and attempt to
B
, which normally recharges during each
IN
decreases to a voltage
B
to recharge.
LTC3836
IN
or tied to
11
3836fa