Datasheet

LTC3562
9
3562fa
OPERATION
Introduction
The LTC3562 is a highly integrated power management
IC that contains four I
2
C controllable, monolithic, high ef-
fi ciency step-down regulators. Two regulators provide up
to 600mA of output current and the other two regulators
produce up to 400mA. All four regulators are 2.25MHz,
constant-frequency, current mode switching regulators that
can be independently controlled through I
2
C. All regula-
tors are internally compensated eliminating the need for
external compensation components.
The LTC3562 offers two different types of adjustable
step-down regulators. The two Type-A regulators (R600A,
R400A) can have the feedback voltages adjusted through
I
2
C from 425mV to 800mV in 25mV increments. The two
Type-B regulators (R600B, R400B) can have the output
voltages adjusted through I
2
C control from 600mV to
3.775V in 25mV increments.
All four converters support 100% duty cycle operation
(low dropout mode) when their input voltage drops very
close to their output voltage. To suit a variety of applica-
tions, four selectable mode functions are available on
the LTC3562’s step-down regulators to trade-off noise
for effi ciency.
At moderate to heavy loads, the constant-frequency pulse
skip mode provides the lowest output switching noise solu-
tion. At lighter loads, either Burst Mode operation, forced
Burst Mode operation or LDO mode may be selected to
optimize effi ciency. The switching regulators also include
soft-start to limit inrush current when powering on, short-
circuit current protection, and switch node slew limiting
circuitry to reduce radiated EMI. No external compensation
components are required.
V
FB
Adjustable (Type-A) Regulators
The two Type-A step-down regulators (R600A and R400A)
have individual programmable feedback servo voltages via
I
2
C control. Given a particular feedback servo voltage, the
output voltage is programmed using a resistor divider from
the switching regulator output connected to the feedback
pins (Figure 1). The output voltage is related to the feedback
servo voltage by the following equation:
V
OUTxA
= V
FBxA
R1
R
2
+ 1






Through I
2
C control, V
FBxA
can be programmed from
800mV (full scale) down to 425mV in 25mV increments.
When the RUN pins (RUN600A and RUN400A) are used
to activate these regulators, the default feedback servo
voltage is set to 800mV.
Typical values for R2 are in the range of 40k to 1MΩ. The
capacitor C
FB
cancels the pole created by the feedback
resistors and the input capacitance of the FB pin and also
helps to improve transient response for output voltages
much greater than 0.8V. A variety of capacitor sizes can be
used for C
FB
but a value of 10pF is recommended for most
applications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
Regulators R600A and R400A have individual RUN pins
that can enable the regulators without accessing the I
2
C
port. The RUN600A and RUN400A pins are OR’ed with the
enable signals coming from the I
2
C port (refer to the Block
Diagram) such that regulators R600A and R400A can be
enabled if the I
2
C port is unavailable. The RUN600A pin is
active low and the RUN400A pin is active high.
When the RUN pins are activated, the Type-A regulators
are enabled in a default setting. The default mode for the
regulators is pulse skip mode and the default feedback
servo voltage setting is 800mV. Once enabled with these
default settings, the settings can always be changed on
the fl y through I
2
C once the I
2
C terminal is available.
The maximum operating output current of regulators R600A
and R400A are 600mA and 400mA, respectively.
Figure 1. Type-A Regulator Application Circuit
R1
C
FB
L
SWxA
FBxA
425mV to 800mV
GND
LTC3562
3562 F01
C
O
R2