Datasheet

LTC3809
10
3809fc
OPERATION
(Refer to Functional Diagram)
Frequency Selection and Phase-Locked Loop
(PLLLPF and SYNC/MODE Pins)
The selection of switching frequency is a tradeoff 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 maintain low output ripple voltage.
The switching frequency of the LTC3809’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 fl oated, tied to V
IN
or tied to GND to select 550kHz,
750kHz or 300kHz, respectively.
A phase-locked loop (PLL) is available on the LTC3809
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 fi lter. The LTC3809
phase detector adjusts the voltage on the PLLLPF pin to
align the turn-on of the external P-channel MOSFET to
the rising edge of the synchronizing signal.
The typical capture range of the LTC3809’s phase-locked
loop is from approximately 200kHz to 1MHz.
Spread Spectrum Modulation (SYNC/MODE and
PLLLPF Pins)
Connecting the SYNC/MODE pin to a DC voltage above
1.35V and several hundred mV below V
IN
enables spread
spectrum modulation (SSM) operation. An internal 2.6μA
pull-down current source at SYNC/MODE helps to set the
voltage at the SYNC/MODE pin for this operation by tying a
resistor with appropriate value between SYNC/MODE and
V
IN
. This mode of operation spreads the internal oscillator
frequency f
OSC
(= 550kHz) over a wider range (460kHz to
635kHz), reducing the peaks of the harmonic output on a
spectral analysis of the output noise. In this case, a 2.2nF
fi lter cap should be connected between the PLLLPF pin
and GND and another 1000pF cap should be connected
between PLLLPF and the SYNC/MODE pin. The controller
operates in PWM pulse-skipping mode at light loads when
spread spectrum modulation is selected. See the discussion
of Spread Spectrum Modulation with SYNC/MODE and
PLLLPF Pins in the Applications Information section.
Dropout Operation
When the input supply voltage (V
IN
) approaches the output
voltage, the rate of change of the inductor current while the
external P-channel MOSFET is on (ON cycle) decreases.
This reduction means that the P-channel MOSFET will
remain on for more than one oscillator cycle if the inductor
current has not ramped up to the threshold set by the
EAMP on the I
TH
pin. Further reduction in the input supply
voltage will eventually cause the P-channel MOSFET to be
turned on 100%; i.e., DC. The output voltage will then be
determined by the input voltage minus the voltage drop
across the P-channel MOSFET and the inductor.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below
safe input voltage levels, an undervoltage lockout is
incorporated in the LTC3809. When the input supply
voltage (V
IN
) drops below 2.25V, the external P- and
N-channel MOSFETs and all internal circuits are turned
off except for the undervoltage block, which draws only
a few microamperes.