LTC3456_15 Datasheet, PDF (15/28 Pages) Linear Technology – 2-Cell, Multi-Output DC/DC Converter with USB Power Manager

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OPERATIO
MAIN REGULATOR
V_INT Output
The main regulator produces a fixed 3.3V output from a
1.8V to 3.2V input (2-cell battery), USB port or AC adapter
input supply. The main regulator output, V_INT, is used to
power most of the internal circuitry of the IC. It is the first
one to power-up. Connect a 22µF or higher X5R or X7R
type ceramic capacitor from this pin to ground.
The loading on this output should be limited to 20mA.
Refer to the Minimum Start-Up Battery Voltage vs
V_INT Output Current graph in the Typical Performance
Characteristics.
When the IC is turned off, the V_INT output voltage is
discharged to ground.
Output Disconnect and Inrush Limiting
The LTC3456 allows true output disconnect when pow-
ered off the battery (boost topology). It achieves this by
disconnecting the body diode of the synchronous PMOS
switch from the output. This allows the V_INT to go to
ground during shutdown. Do not connect a Schottky diode
from SW2_BST to V_INT; doing so will defeat the output
disconnect feature.
The LTC3456 also features inrush current limiting at
power-up (battery powered). Inrush current in boost
converters is important when powering from input sources
with high input impedance like alkaline cells. The LTC3456
incorporates an inrush current limiting scheme that regu-
lates the inrush current to 600mA (typ) during power-up.
Figure 5 shows inrush current when the device is powered
up from the battery.
Short-Circuit Protection
The LTC3456 features short-circuit protection for the main
regulator output. When the main regulator is powered
from the USB or wall input, it operates in a buck topology.
LTC3456
PWRON
5V/DIV
V_INT
2V/DIV
I_L2
500mA/DIV
V_BATT = 2.4V
I_VINT = 10mA
100µs/DIV
3456 F05
Figure 5. Inrush Current at Power-Up (Battery Powered)
If the main regulator outputs (V_INT or V_MAIN) are shorted
to ground, the LTC3456 unique control scheme prevents
inductor current runaway.
When the device is powered from the battery, it operates
in a boost topology. Most boost converters do not allow their
outputs to be shorted to ground. However, the LTC3456
allows the output of its main regulator (V_INT or V_MAIN) to
be short-circuited due to its unique inrush current limiting.
In the event of a short-circuit, the input current is well
regulated.
V_MAIN Output
The LTC3456 is designed to supply power to the micro-
processor peripheral circuitry in a controlled manner. The
peripheral circuitry should be connected to the V_MAIN
output.
V_MAIN is connected to V_INT via a 0.4Ω (typ) PMOS switch
0.8ms (typ) after the Core output comes into regulation.
This ensures that the peripheral circuitry always powers
up after the microprocessor.
The V_MAIN output is discharged to ground through inter-
nal pull-down resistors at shutdown. This ensures that
the peripheral circuitry gets turned-off completely during
shutdown. Connect a 1µF (X5R or X7R) bypass capacitor
from this pin to ground.
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