Datasheet

LTC3852
14
3852f
inductor current reaches zero, preventing it from revers-
ing and going negative. Thus, the controller operates in
discontinuous operation. In forced continuous operation,
the inductor current is allowed to reverse at light loads
or under large transient conditions. The peak inductor
cur rent is determined by the voltage on the I
TH
pin, just
as in normal operation. In this mode the effi ciency at light
loads is lower than in Burst Mode operation. However,
continuous mode has the advantages of lower output
ripple and constant frequency operation.
When the MODE/PLLIN pin is connected to GND2, the
LTC3852 operates in PWM pulse skipping mode at light
loads. At very light loads the current comparator, I
CMP
, may
remain tripped for several cycles and force the external top
MOSFET to stay off for the same number of cycles (i.e.,
skipping pulses). The inductor current is not allowed to
reverse (discontinuous operation). This mode, like forced
continuous operation, exhibits low output ripple as well as
low audio noise and reduced RF interference as compared
to Burst Mode operation. It provides higher low current
effi ciency than forced continuous mode, but not as high
as Burst Mode operation.
Frequency Selection and Phase-Locked Loop
(FREQ/PLLFLTR and MODE/PLLIN Pins)
The selection of a switching frequency is a trade-off between
effi ciency and component size. Low frequency operation
increases effi ciency by reducing MOSFET switching losses, but
requires larger inductance and/or capacitance to main tain low
output ripple voltage. The switching frequency of the LTC3852’s
controller can be selected using the FREQ/PLLFLTR pin. If
the MODE/PLLIN pin is not being driven by an external clock
source, the FREQ/PLLFLTR pin can be used to program the
controller’s operating frequency from 250kHz to 750kHz.
A phase-locked loop (PLL) is available on the LTC3852
to synchronize the internal oscillator to an external clock
source that is connected to the MODE/PLLIN pin. The
controller operates in forced continuous mode of operation
when it is synchronized. A series RC should be connected
between the FREQ/PLLFLTR pin and GND to serve as the
PLL’s loop fi lter.
It is suggested that the external clock be applied before
enabling the controller unless a second resistor is
OPERATION
connected in parallel with the series RC loop fi lter network.
The second resistor prevents low switching frequency
operation if the controller is enabled before the clock.
Output Overvoltage Protection
An overvoltage comparator, OV, guards against transient
overshoots (>10%) as well as other more serious con-
ditions that may overvoltage the output of the step-down
controller. In such cases, the top MOSFET is turned off
and the bottom MOSFET is turned on until the overvoltage
condition is cleared.
Power Good (PGOOD) Pin
The PGOOD pin is connected to an open drain of an internal
N-channel MOSFET. The MOSFET turns on and pulls the
PGOOD pin low when the V
FB
pin voltage is not within
±10% of the 0.8V reference voltage. The PGOOD pin is
also pulled low when the RUN pin is low (shut down)
or when the LTC3852’s controller is in the soft-start or
tracking phase. When the V
FB
pin voltage is within the
±10% requirement, the MOSFET is turned off and the
pin is allowed to be pulled up by an external resistor to
a source of up to 6V (abs max). The PGOOD pin will fl ag
power good immediately when the V
FB
pin is within the
±10% window. However, there is an internal 17µs power
bad mask when V
FB
goes out of the ±10% window.
Short-Circuit/Thermal Protection
The charge pump has built-in short-circuit current limit as
well as over-temperature protection. During a short-circuit
condition, it will automatically limit V
PUMP
output current
to approximately 300mA. At higher temperatures, or if the
input voltage is high enough to cause excessive self-heating
of the part, the thermal shutdown circuitry will shut down
the charge pump once the junction temperature exceeds
approximately 160°C. It will enable the charge pump once its
junction temperature drops back to approximately 150°C.
The charge pump will cycle in and out of thermal shutdown
indefi nitely until the short-circuit condition on V
PUMP
is
removed. The maximum rated junction temperature will
be exceeded when this thermal shutdown protection is
active. Continuous operation above the specifi ed absolute
maximum operating junction temperature may impair
device reliability or permanently damage the device.