LTC3577-4_15 Datasheet, PDF (29/52 Pages) Linear Technology – Highly Integrated Portable Product PMIC

LTC3577-3/LTC3577-4
OPERATION
When disabled all LDO circuitry is powered off leaving
only a few nanoamps of leakage current on the LDO sup-
ply. Both LDO outputs are individually pulled to ground
through internal resistors when disabled.
The power good status bits of LDO1 and LDO2 are avail-
able in I²C through the read-back registers PGLDO[1] and
PGLDO[2] for LDO1 and LDO2 respectively. The power
good comparators for both LDOs are sampled when the
I²C port receives the correct I²C read address.
Figure 7 shows the LDO application circuit. The full-scale
output voltage for each LDO is programmed using a resistor
divider from the LDO output (LDO1 or LDO2) connected
to the feedback pins (LDO1_FB or LDO2_FB) such that:
V_LDOx
=
0.8V
•
⎛
⎝⎜
R1
R2
+
1^{⎞⎠⎟}
For stability, each LDO output must be bypassed to ground
with a minimum 1μF ceramic capacitor (C_OUT).
V_INLDOx
LDOxEN ⁰
1
MP
LDOx
R1
LDOx_FB
0.8V
GND
R2
LDOx
OUTPUT
C_OUT
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Figure 7. LDO Application Circuit
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 the “Pushbutton Interface” sec-
tion for more information), while Buck3 has an individual
enable 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.
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