LTC3555 Linear Technology, LTC3555 Datasheet - Page 16

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

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LTC3555
OPERATION
at V
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
conductance is needed, an external P-channel MOSFET
transistor can be added from BAT to V
When an external P-channel MOSFET transistor is present,
the GATE pin of the LTC3555 drives its gate for automatic
ideal diode control. The source of the external P-channel
MOSFET should be connected to V
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 40mΩ or lower.
Suspend LDO
If the LTC3555 is confi gured for USB suspend mode,
the switching regulator is disabled and the suspend LDO
provides power to the V
available to V
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 specifi ca-
tion, the input to the LDO is current limited so that it will
not exceed the 500μA low power suspend specifi cation.
16
OR WALL
ADAPTER
TO USB
OUT
is approximately 15mV (V
BUS
25
2
I
h
SWITCH
V
CLPROG
CLPROG
BUS
). This LDO will prevent the battery from
1.188V
/
AVERAGE INPUT
CURRENT LIMIT
OUT
CONTROLLER
–
+
pin (presuming there is power
GATE DRIVE
PWM AND
FWD
OUT
) below the voltage at
and the drain should
OUT
AVERAGE OUTPUT
VOLTAGE LIMIT
CONTROLLER
.
Figure 3. PowerPath Block Diagram
–
+
+
3.6V
CONSTANT CURRENT
CONSTANT VOLTAGE
BATTERY CHARGER
0.3V
+ –
If the load on V
the additional current will come from the battery via the
ideal diode.
3.3V Always-On LDO Supply
The LTC3555 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
dropout at loads less than 25mA as V
If the LDO3V3 output is not used, it should be disabled
by connecting it to V
V
An internal undervoltage lockout circuit monitors V
keeps the PowerPath switching regulator off until V
rises above 4.30V and is about 200mV above the battery
voltage. Hysteresis on the UVLO turns off the regulator if
V
this happens, system power at V
the battery via the ideal diode.
BUS
BUS
drops below 4.00V or to within 50mV of BAT. When
Undervoltage Lockout (UVLO)
IDEAL
DIODE
15mV
–
+
OUT
+
–
OUT
exceeds the suspend current limit,
GATE
V
.
BAT
OUT
SW
26
24
22
23
OUT
3555 F03
, and therefore will enter
OUT
will be drawn from
OUT
+
SINGLE CELL
Li-Ion
falls near 3.3V.
OPTIONAL
EXTERNAL
IDEAL DIODE
PMOS
3.5V TO
(BAT + 0.3V)
TO SYSTEM
LOAD
BUS
and
BUS
3555f