LTC3822-1 Datasheet, PDF (10/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous Step-Down DC/DC Controller

LTC3822-1
OPERATION (Refer to Functional Diagram)
signal goes low and the controller resumes normal opera-
tion by turning on the top N-channel MOSFET on the next
cycle of the internal oscillator.
When the controller is enabled for Burst Mode or pulse
skipping operation, the inductor current is not allowed to
reverse. Hence, the controller operates discontinuously.
The reverse current comparator RICMP senses the drain-
to-source voltage of the bottom N-channel MOSFET. This
MOSFET is turned off just before the inductor current
reaches zero, preventing it from going negative.
In forced continuous operation, the inductor current is
allowed to reverse at light loads or under large transient
conditions. The peak inductor current is determined by
the voltage on the I_TH pin. The top MOSFET is turned on
every cycle (constant frequency) regardless of the I_TH pin
voltage. In this mode, the efficiency at light loads is lower
than in Burst Mode operation. However, continuous mode
has the advantages of lower output ripple and no noise at
audio frequencies.
When the SYNC/MODE pin is clocked by an external clock
source to use the phase-locked loop (see Frequency Selec-
tion and Phase-Locked Loop), or is set to a DC voltage
between 0.4V and several hundred mV below V_IN, the
LTC3822-1 operates in PWM pulse skipping mode at light
loads. In this mode, the current comparator ICMP may
remain tripped for several cycles and force the external top
MOSFET to stay off for the same number of cycles. 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 op-
eration. However, it provides low current efficiency higher
than forced continuous mode, but not nearly as high as
Burst Mode operation. During start-up or an undervoltage
condition (V_FB ≤ 0.54V), the LTC3822-1 operates in pulse
skipping mode (no current reversal allowed), regardless
of the state of the SYNC/MODE pin.
Short-Circuit Protection
The LTC3822-1 monitors V_FB to detect a short-circuit
on V_OUT. When V_FB is near ground, switching frequency
is reduced to prevent the inductor current from running
away. The oscillator frequency will progressively return
10
to normal when V_FB rises above ground. This feature is
disabled during start-up.
Output Overvoltage Protection
As further protection, the overvoltage comparator (OVP)
guards against transient overshoots, as well as other more
serious conditions that may overvoltage the output. When
the feedback voltage on the V_FB pin has risen 13.33%
above the reference voltage of 0.6V, the top MOSFET is
turned off and the bottom MOSFET is turned on until the
overvoltage is cleared.
Frequency Selection and Phase-Locked Loop
(PLLLPF and SYNC/MODE Pins)
The selection of switching frequency is a tradeoff 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 frequency of the LTC3822-1’s controllers
can be selected using the PLLLPF pin. If the SYNC/MODE
is not being driven by an external clock source, the PLLLPF
can be floated, tied to V_IN or tied to GND to select 550kHz,
750kHz or 300kHz, respectively.
A phase-locked loop (PLL) is available on the LTC3822-1
to synchronize the internal oscillator to an external clock
source that connects to the SYNC/MODE pin. In this case,
a series RC should be connected between the PLLLPF pin
and GND to serve as the PLL’s loop filter. The LTC3822-1
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of the top MOSFET to the rising edge of
the synchronizing signal.
The typical capture range of the LTC3822-1’s phase-locked
loop is from approximately 200kHz to 1MHz.
Boost Capacitor Refresh Timeout
In order to maintain sufficient charge on C_B, the converter
will briefly turn off the top MOSFET and turn on the bottom
MOSFET if at any time the bottom MOSFET has remained
off for 10 cycles. This most commonly occurs in a dropout
situation where V_IN is close to V_OUT.
38221f