LTC3852
12
3852f
OPERATION
Main Control Loop
The LTC3852 is a constant frequency, current mode
step-down DC/DC controller which can be powered by
an onboard charge pump. Supplies as low as 2.7V, when
doubled by the charge pump, provide 5V to the LTC3852’s
control logic and gate drives, supporting a wide selection
of logic-level MOSFETs.
During normal operation, the controller’s top MOSFET is
turned on when the clock sets the RS latch, and is turned
off when the main current comparator, I
CMP
, resets the
RS latch. The peak inductor current at which I
CMP
resets
the RS latch is controlled by the voltage on the I
TH
pin,
which is the output of the error ampli fi er EA. The V
FB
pin
receives the voltage feedback signal, which is compared
to the internal reference voltage by the EA. When the
load current increases, it causes a slight decrease in V
FB
relative to the 0.8V reference, which in turn causes the
I
TH
voltage to increase until the average inductor current
matches the new load current. After the top MOSFET has
turned off, the bottom MOSFET is turned on until either
the inductor current starts to reverse, as indicated by the
reverse current comparator, I
REV
, or the beginning of the
next cycle.
The charge pump section uses a switched capacitor doubler
to boost V
IN1
to 2 ¥ V
IN1
, with a regulated maximum of
5V. Regulation is achieved by sensing the output voltage
through an internal resistor divider and modulating the
charge pump output current based on the error signal. A
2-phase nonoverlapping clock activates the charge pump
switches. The fl ying capacitor is charged from V
IN1
on
the fi rst phase of the clock. On the second phase of the
clock it is stacked in series with V
IN1
and connected to
V
PUMP
. This sequence of charging and discharging the
fl ying capacitor continues at a free running frequency of
1.2MHz (typ).
Two confi gurations address most LTC3852 applications.
Figure 1a covers the single low input voltage case, typically
3.3V. The input to the charge pump, V
IN1
, is connected to
the same input voltage as the drain of the top MOSFET.
V
PUMP
, V
IN2
and INTV
CC
are tied together, so that the
charge pump’s 5V output provides all power to the buck
controller section.
The alternative arrangement in Figure 1b allows the
LTC3852 to step down from as high as 38V, while powering
itself from an available 3.3V bus. The input to the charge
pump, V
IN1
, is connected to 3.3V instead of the drain of
the top MOSFET. It is not necessary to step the high input
voltage down to 5V through a linear regulator. Logic level
MOSFETs are usable in both cases.
V
IN
3.3V
3852 F01a
V
IN1
V
PUMP
V
IN2
INTV
CC
TG
BG
Figure 1a
V
IN
≤ 38V
3852 F01b
V
IN1
V
PUMP
V
IN2
INTV
CC
TG
BG
3.3V
Figure 1b
INTV
CC
Power
Power for the top and bottom MOSFET drivers and most
other internal circuitry is derived from the INTV
CC
pin. Peak
current drawn from INTV
CC
should not exceed 50mA.
The top MOSFET driver is biased from the fl oating boot-
strap capacitor, C
B
, which normally recharges during each
off cycle through an external diode when the top MOSFET
turns off. If V
IN
decreases to a voltage close to V
OUT
, the
loop may enter dropout and attempt to turn on the top
MOSFET continuously. The dropout detec tor detects this
and forces the top MOSFET off for about 1/10 of the clock
period every tenth cycle to allow C
B
to recharge. However, it
