Datasheet

LTC3566/LTC3566-2
13
3566fb
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 confi gured 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.
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 effi ciency 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 amplifi er 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 suffi cient for the application, then
no external components are necessary. However, if more
OPERATION
BAT (V)
2.4
4.5
4.2
3.9
3.6
3.3
3.0
2.7
2.4
3.3 3.9
3566 F01
2.7 3.0
3.6 4.2
V
OUT
(V)
NO LOAD
300mV
Figure 1. V
OUT
vs BAT
Figure 2. Ideal Diode Operation
FORWARD VOLTAGE (mV) (BAT – V
OUT
)
0
CURRENT (mA)
600
1800
2000
2200
120
240
300
3566 F02
200
1400
1000
400
1600
0
1200
800
60
180
360
480420
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
LTC3566
IDEAL DIODE
ON
SEMICONDUCTOR
MBRM120LT3
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