LTC3569_15 Datasheet, PDF (11/26 Pages) Linear Technology – Triple Buck Regulator with 1.2A and Two 600mA Outputs and Individual Programmable References

LTC3569
Operation
(NCOMP) signals to turn-off the N-channel switch, so
that is does not discharge the output capacitor. When a
rising clock edge occurs, the P-channel switch turns on
repeating the cycle.
The peak inductor current is controlled by the error amplifier
(EA) and is influenced by the slope compensation. The error
amplifier compares the FB pin voltage to the programmed
internal reference (REF). When the load current increases,
the FB voltage decreases. When the FB voltage falls below
the reference voltage, the error amplifier output rises
to increase the peak inductor current until the average
inductor current matches the new load current. With the
inductor current equal to the load current, the duty cycle
will stabilize to a value equal to V_OUT/V_IN.
Low Current Operation
At light loads, the FB voltage may rise above the refer-
ence voltage. If this occurs the error amplifier signals
the control loop to go to sleep, and the P-channel turns
off immediately. The inductor current then discharges
through the N-channel switch until the inductor current
approaches zero; whereupon the SW goes Hi-Z, and the
output capacitor supplies power to the load. When the
load discharges the output capacitor the feedback voltage
falls and the error amp wakes up the buck, restarting the
main control loop as if a clock cycle has just begun. This
sleep cycle helps minimize the switching losses which are
dominated by the gate charge losses of the power devices.
Two operating modes are available to control the operation
of the LTC3569 at low currents, Burst Mode operation and
pulse-skipping mode.
Select Burst Mode operation to optimize efficiency at low
output currents. In Burst Mode operation the inductor cur-
rent reaches a fixed current before the P-channel switch
compares inductor current against the value determined
by I_TH. This burst clamp causes the output voltage to rise
above the regulation voltage and forces a longer sleep
cycle. This greatly reduces switching losses and aver-
age quiescent current at light loads, at the cost of higher
ripple voltage.
Pulse-skipping mode is intended for lower output voltage
ripple at light load currents. Here, the peak P-channel cur-
rent is compared with the value determined by the error
amplifier output. Then, the P-channel is turned off and the
N-channel switch is turned on until either the next cycle
begins or the N-channel comparator (NCOMP) turns off the
N-channel switch. If the NCOMP trips, the SW node goes
Hi-Z and the buck operates discontinuously. In pulse-skip-
ping mode the LTC3569 continues to switch at a constant
frequency down to very low currents; where it eventually
begins skipping pulses. Because the LTC3569 remains
active at lighter load currents in pulse-skipping mode, the
efficiency performance is traded off against output voltage
ripple and electromagnetic interference (EMI).
Dropout Operation
When the input supply voltage decreases towards the out-
put voltage the duty cycle automatically increases to 100%;
which is the dropout condition. In dropout, the P-channel
switch is turned on continuously with the output voltage
being equal to the input voltage minus the voltage drop
across the internal P-channel switch and the inductor.
Low Supply Operation
The LTC3569 incorporates an undervoltage lockout circuit
which shuts down the part when the input voltage drops
below 2.5V to prevent unstable operation. The UVLO
function does not reset the reference voltage DAC. (See
Programming the Reference.)
Slave Power Stage
When the FB pin of one of the two 600mA regulators is tied
to SV_IN that regulator’s control circuits are disabled and
the regulator’s switch pin is configured to follow a master
regulator; either the first 600mA regulator (regulator 2) or
the 1.2A regulator (regulator 1). In this way, two regula-
tor power stages are ganged together (e.g., switch pins
shorted together to a single inductor) to support higher
current levels. This permits three permutations of power
levels: three independent regulators at 1.2A, 600mA and
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