LTC3802
12
3802f
APPLICATIO S I FOR ATIO
WUUU
Switching Architecture
The LTC3802 includes two step-down (buck) voltage
mode feedback switching regulator controllers. These two
controllers act independently of each other except at start-
up and current limit. For proper power-up sequencing,
channel 1 is designated to be the higher output voltage
channel (see Start-Up Tracking).
Each channel uses two external sychronous N-channel
MOSFETs. A floating topside driver and a simple external
charge pump provide full gate drive to each upper MOSFET.
The controller uses leading edge modulation architecture
to allow extremely low duty cycle and fast load recovery
operation. In a typical LTC3802 switching cycle, the PWM
comparator turns on the top MOSFET and charges up the
output capacitor. Some time later, an internal clock resets
the top MOSFET, turns on the bottom MOSFET and re-
duces the output charging current. The top gate duty cycle
is controlled by the feedback amplifier, which compares
the divided output voltage with an internal reference. This
switching cycle repeats itself at a fixed 550kHz frequency
or in synchronization with an external oscillator.
The internal master clock runs at 550kHz, turning off the
top gate once every 1.8µs. Thus, feedback loop compo-
nents and output inductors and capacitors can be scaled
to a particular operating frequency. Noise generated by the
circuit will always be in a known frequency band, with the
550kHz frequency designed to leave the 455kHz IF band
free of interference. Subharmonic oscillation and slope
compensation, common headaches with constant fre-
quency current mode switchers, are absent in voltage
mode designs like the LTC3802. Two LTC3802 channels
run from a common clock, with the phasing chosen to be
180° from channel 1 to channel 2. This has the effect of
doubling the frequency of the switching pulses seen by the
input bypass capacitor, significantly lowering its RMS
current and reducing the capacitance required.
Feedback Control
Each LTC3802 channel senses the output voltage at V
OUT
with an internal feedback op amp (see Block Diagram).
This is a real op amp with a low impedance output, 80dB
open-loop gain and 10MHz gain-bandwidth product. The
positive input is connected to a level-shifted internal
600mV reference, while the negative input is connected to
the level-shifted FB pin. The output is connected to COMP,
which is in turn connected to the line feedforward circuit
and from there to the PWM generator. To speed up the
overshoot recovery time, the maximum potential at the
COMP pin is internally clamped at a level corresponding to
the maximum top gate duty cycle. Under start-up condi-
tions, RUN/SS controls the COMP pin slew rate.
At steady state, as shown in Figure 1, the output of the
switching regulator is given the following equation
VV
R
R
OUT REF
B
=+
•1
1
Unlike many regulators that use a transconductance (g
m
)
amplifier, the LTC3802 is designed to use an inverting sum-
ming amplifier topology with the FB pin configured as a
virtual ground. This allows the feedback gain to be tightly
controlled by external components, which is not possible
with a simple g
m
amplifier. In addition, the voltage feed-
back amplifier allows flexibility in choosing pole and zero
locations. In particular, it allows the use of “Type 3” com-
pensation, which provides a phase boost at the LC pole
frequency and significantly improves the control loop phase
margin.
In a typical LTC3802 circuit, the feedback loop consists of
the line feedforward circuit, the modulator, the external
inductor, the output capacitor and the feedback amplifier
with its compensation network. All these components
affect loop behavior and need to be accounted for in the
loop compensation. The modulator consists of the PWM
generator, the output MOSFET drivers and the external
MOSFETs themselves. The modulator gain varies linearily
with the input voltage. The line feedforward circuit com-
pensates for this change in gain, and provides a constant
gain from the error amplifier output to the inductor input
regardless of input voltage. From a feedback loop point of
view, the combination of the line feedforward circuit and
the modulator looks like a linear voltage transfer function
from COMP to the inductor input and has a gain roughly
equal to 22V/V. It has fairly benign AC behavior at typical
loop compensation frequencies with significant phase
shift appearing at half the switching frequency.
