LTC3851A_15 Datasheet, PDF (11/30 Pages) Linear Technology – Synchronous Step-Down Switching Regulator Controller

LTC3851A
Operation
the error amplifier, EA, will decrease the voltage on the I_TH
pin. When the I_TH voltage drops below 0.4V, the internal
sleep signal goes high (enabling sleep mode) and both
external MOSFETs are turned off.
In sleep mode, the load current is supplied by the output
capacitor. As the output voltage decreases, the EA’s output
begins to rise. When the output voltage drops enough, the
sleep signal goes low, and the controller resumes normal
operation by turning on the top external MOSFET on the
next cycle of the internal oscillator. When a controller is
enabled for Burst Mode operation, the inductor current is
not allowed to reverse. The reverse current comparator,
I_REV , turns off the bottom external MOSFET just before the
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 efficiency at light loads
is lower than in Burst Mode operation. However, continu-
ous mode has the advantages of lower output ripple and
less interference to audio circuitry.
When the MODE/PLLIN pin is connected to GND, the
LTC3851A 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
efficiency than forced continuous mode, but not nearly as
high as Burst Mode operation.
Frequency Selection and Phase-Locked Loop
(FREQ/PLLFLTR and MODE/PLLIN Pins)
The selection of switching frequency is a trade-off between
efficiency and component size. Low frequency opera-
tion increases efficiency by reducing MOSFET switching
losses, but requires larger inductance and/or capacitance
to main­tain low output ripple voltage. The switching fre-
quency of the LTC3851A 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 LTC3851A
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 filter. It is suggested that the external clock be
applied before enabling the controller unless a second
resistor is connected in parallel with the series RC network.
The second resistor prevents very 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. In such cases,
the top MOSFET is turned off and the bottom MOSFET is
turned on until the overvoltage condition is cleared.
3851afa
11