LTC3566-2_15 Datasheet, PDF (13/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566/LTC3566-2
OPERATION
the programmed limit, CLPROG reaches V_CLPROG, 1.188V
and power out is held constant.
The input current is programmed by the ILIM0 and ILIM1
pins. It can be configured to limit average input current to
one of several possible settings as well as be deactivated
(USB Suspend). The input current limit will be set by the
V_CLPROG servo voltage and the resistor on CLPROG ac-
cording to the following expression:
I_VBUS
= I_BUSQ
+
V_CLPROG
R_CLPROG
• (h_CLPROG
+
1)
Figure 1 shows the range of possible voltages at V_OUT as
a function of battery voltage.
4.5
4.2
3.9
NO LOAD
3.6
3.3
300mV
3.0
2.7
2.4
2.4
2.7 3.0 3.3 3.6 3.9 4.2
BAT (V)
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Figure 1. V_OUT vs BAT
The LTC3566 Versus the LTC3566-2
For very low battery voltages, the battery charger acts
like a load and, due to limited input power, its current
can cause V_OUT to approach the battery voltage when
operating under input current limit conditions. To prevent
V_OUT from falling to this level, the LTC3566-2 includes an
undervoltage circuit that automatically detects that V_OUT
is falling and reduces the battery charge current as V_OUT
falls from 3.5V to 3.3V. This reduction prevents V_OUT from
collapsing suddenly towards the battery voltage when input
current limit is reached and ensures that load current and
output voltage are always prioritized while delivering as
much battery charge current as possible. The standard
LTC3566 does not include this circuit and thus favors
maximum charge current at all times over output voltage
preservation. See graph titled Output Current vs Output
2200
2000
1800
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
1600
1400
1200
LTC3566
IDEAL DIODE
1000
800
600
ON
400
SEMICONDUCTOR
MBRM120LT3
200
0
0 60 120 180 240 300 360 420 480
FORWARD VOLTAGE (mV) (BAT – V_OUT)
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Figure 2. Ideal Diode Operation
Voltage (Battery Charger Enabled) 5x Mode in the Typical
Performance Characteristics section.
If instant-on operation under low battery and input current
limited conditions is a requirement, then the LTC3566-2
should be used. If maximum charge efficiency at low
battery voltages is preferred, and instant-on operation is
not a requirement, then the standard LTC3566 should be
selected. All versions of the LTC3566 family will start up
with a removed battery.
Ideal Diode from BAT to V_OUT
The LTC3566 family has an internal ideal diode as well as
a controller for an optional external ideal diode. The ideal
diode controller is always on and will respond quickly
whenever V_OUT drops below BAT.
If the load current increases beyond the power allowed
from the switching regulator, additional power will be
pulled from the battery via the ideal diode. Furthermore,
if power to V_BUS (USB or wall power) is removed, then all
of the application power will be provided by the battery via
the ideal diode. The transition from input power to battery
power at V_OUT will be quick enough to allow only a 10μF
capacitor to keep V_OUT from drooping. The ideal diode
consists of a precision amplifier that enables a large on-
chip P-channel MOSFET transistor whenever the voltage at
V_OUT is approximately 15mV (V_FWD) below the voltage at
BAT. The resistance of the internal ideal diode is approxi-
mately 180mΩ. If this is sufficient for the application, then
no external components are necessary. However, if more
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