Datasheet

LTC3556
16
3556f
OPERATION
V
OUT
is approximately 15mV (V
FWD
) below the voltage at
BAT. The resistance of the internal ideal diode is approxi-
mately 180m. If this is sufficient for the application, then
no external components are necessary. However, if more
conductance is needed, an external P-channel MOSFET
transistor can be added from BAT to V
OUT
.
When an external P-channel MOSFET transistor is present,
the GATE pin of the LTC3556 drives its gate for automatic
ideal diode control. The source of the external P-chan-
nel MOSFET should be connected to V
OUT
and the drain
should be connected to BAT. Capable of driving a 1nF load,
the GATE pin can control an external P-channel MOSFET
transistor having an on-resistance of 40m or lower.
Suspend LDO
If the LTC3556 is configured for USB suspend mode, the
switching regulator is disabled and the suspend LDO
provides power to the V
OUT
pin (presuming there is power
available to V
BUS
). This LDO will prevent the battery from
running down when the portable product has access to
a suspended USB port. Regulating at 4.6V, this LDO only
becomes active when the switching converter is disabled
(suspended). To remain compliant with the USB specifica-
tion, the input to the LDO is current limited so that it will
not exceed the 500µA low power suspend specification.
If the load on V
OUT
exceeds the suspend current limit,
the additional current will come from the battery via the
ideal diode.
3.3V Always-On Supply
The LTC3556 includes a low quiescent current low dropout
regulator that is always powered. This LDO can be used to
provide power to a system pushbutton controller, standby
microcontroller or real time clock. Designed to deliver up
to 25mA, the always-on LDO requires at least a 1µF low
impedance ceramic bypass capacitor for compensation.
The LDO is powered from V
OUT
, and therefore will enter
dropout at loads less than 25mA as V
OUT
falls near 3.3V.
If the LDO3V3 output is not used, it should be disabled
by connecting it to V
OUT
.
V
BUS
Undervoltage Lockout (UVLO)
An internal undervoltage lockout circuit monitors V
BUS
and
keeps the PowerPath switching regulator off until V
BUS
rises above 4.30V and is about 200mV above the battery
voltage. Hysteresis on the UVLO turns off the regulator if
V
BUS
drops below 4.00V or to within 50mV of BAT. When
this happens, system power at V
OUT
will be drawn from
the battery via the ideal diode.
+
–
+
+
–
0.3V
1.188V 3.6V
CLPROG
I
SWITCH
/
h
CLPROG
+
–
+
–
15mV
IDEAL
DIODE
PWM AND
GATE DRIVE
AVERAGE INPUT
CURRENT LIMIT
CONTROLLER
AVERAGE OUTPUT
VOLTAGE LIMIT
CONTROLLER
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
+
–
2
GATE
22
V
OUT
24
SW
3.5V TO
(BAT + 0.3V)
TO SYSTEM
LOAD
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
SINGLE CELL
Li-Ion
3556 F03
26
BAT
23
V
BUS
TO USB
OR WALL
ADAPTER
25
+
Figure 3. PowerPath Block Diagram