LTC3577-3 Datasheet, PDF (30/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
STEP-DOWN SWITCHING REGULATOR OPERATION
Introduction
The LTC3577-3/LTC3577-4 include three 2.25MHz
constant-frequency current mode step-down switching
regulators providing 500mA, 500mA and 800mA each.
All step-down switching regulators can be programmed
for a minimum output voltage of 0.8V and can be used to
power a microcontroller core, microcontroller I/O, memory
or other logic circuitry. All step-down switching regulators
support 100% duty cycle operation (low dropout mode)
when the input voltage drops very close to the output
voltage and are also capable of Burst Mode operation for
highest efficiencies at light loads. Burst Mode operation
is individually selectable for each step-down switching
regulator through the I²C register bits BK1BRST, BK2BRST
and BK3BRST. The step-down 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 EMI radiation. No external
compensation components are required for the switch-
ing regulators. Switching regulators 1 and 2 (Buck1 and
Buck2) are sequenced up and down together through the
pushbutton interface (see “Pushbutton Interface” section
for more information), while Buck3 has an individual en-
able pin (EN3) that is active when the pushbutton is in
the power up or power on states. Buck3 is disabled in the
power down and power off states. It is recommended that
the step-down switching regulator input supplies (V_IN12
and V_IN3) be connected to the system supply pin (V_OUT).
This is recommended because the undervoltage lockout
circuit on the V_OUT pin (V_OUT UVLO) disables the step-
down switching regulators when the V_OUT voltage drops
below the V_OUT UVLO threshold. If driving the step-down
switching regulator input supplies from a voltage other
than V_OUT the regulators should not be operated outside
the specified operating range as operation is not guaranteed
beyond this range.
Output Voltage Programming
Figure 8 shows the step-down switching regulator ap-
plication circuit. The full-scale output voltage for each
step-down switching regulator is programmed using a
V_IN
EN
MODE _PWM
SLEW CONTROL
GND
MP
SWx
L
MN
C_FB
FBx
0.8V
V_OUTx
R1
C_OUT
R2
357734 F08
Figure 8. Step-Down Switching Regulator Application Circuit
resistor divider from the step-down switching regulator
output connected to the feedback pins (FB1, FB2 and
FB3) such that:
V_OUTx
=
0.8V
•
⎛
⎝⎜
R1
R2
+
⎞
1_⎠⎟
Typical values for R1 are in the range of 40k to 1M. The
capacitor C_FB cancels the pole created by feedback resis-
tors 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 ap-
plications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
Operating Modes
The step-down switching regulators include two possible
operating modes to meet the noise/power needs of a variety
of applications. In pulse-skipping mode, an internal latch
is set at the start of every cycle, which turns on the main
P-channel MOSFET switch. During each cycle, a current
comparator compares the peak inductor current to the
output of an error amplifier. The output of the current
comparator resets the internal latch, which causes the main
P-channel MOSFET switch to turn off and the N-channel
MOSFET synchronous rectifier to turn on. The N-channel
MOSFET synchronous rectifier turns off at the end of the
2.25MHz cycle or if the current through the N-channel
MOSFET synchronous rectifier drops to zero. Using this
method of operation, the error amplifier adjusts the peak
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