LTC3557_15 Datasheet, PDF (13/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
OPERATION
The LTC3557/LTC3557-1 includes three 2.25MHz constant
frequency current mode step-down switching regulators
providing 400mA, 400mA and 600mA each. All step-
down switching regulators can be programmed for a
minimum output voltage of 0.8V and can be used to power
a microcontroller core, microcontroller I/O, memory or
other logic circuitry. All step-down switching regulators
support 100% duty cycle operation and are capable of
operating in Burst Mode operation for highest efficiencies
at light loads (Burst Mode operation is pin selectable). No
external compensation components are required for the
switching regulators.
USB PowerPath Controller
The input current limit and charge control circuits of the
LTC3557/LTC3557-1 are designed to limit input current
as well as control battery charge current as a function of
I_VOUT. V_OUT drives the combination of the external load,
the three step-down switching regulators, always on 3.3V
LDO and the battery charger.
If the combined load does not exceed the programmed
input current limit, V_OUT will be connected to V_BUS through
an internal 200mΩ P-channel MOSFET.
If the combined load at V_OUT exceeds the programmed
input current limit, the battery charger will reduce its charge
current by the amount necessary to enable the external
load to be satisfied while maintaining the programmed
input current. Even if the battery charge current is set to
exceed the allowable USB current, the average input current
USB specification will not be violated. Furthermore, load
current at V_OUT will always be prioritized and only excess
available current will be used to charge the battery.
The current out of the CLPROG pin is a fraction (1/h_CLPROG)
of the V_BUS current. When a programming resistor is con-
nected from CLPROG to GND, the voltage on CLPROG
represents the input current:
I_VBUS
= I_BUSQ
+
V_CLPROG
R_CLPROG
• h_CLPROG
where I_BUSQ and h_CLPROG are given in the Electrical
Characteristics.
The input current limit is programmed by the ILIM0 and
ILIM1 pins. The LTC3557/LTC3557-1 can be configured to
limit input current to one of several possible settings as well
as be deactivated (USB Suspend). The input current limit
will be set by the appropriate servo voltage and the resistor
on CLPROG according to the following expression:
( ) I_VBUS
= I_BUSQ
+
0.2V
R_CLPROG
• h_CLPROG
1x Mode
( ) I_VBUS
= I_BUSQ
+
1V
R_CLPROG
• h_CLPROG
5x Mode
( ) I_VBUS
= I_BUSQ
+
2V
R_CLPROG
• h_CLPROG
10x Mode
Under worst-case conditions, the USB specification will not
be violated with an R_CLPROG resistor of greater than 2.1k.
Table 1 shows the available settings for the ILIM0 and
ILIM1 pins:
Table 1: Controlled Input Current Limit
ILIM1
ILIM0
0
0
0
1
1
0
1
1
I_BUS(LIM)
100mA (1x)
1A (10x)
Suspend
500mA (5x)
Notice that when ILIM0 is high and ILIM1 is low, the input
current limit is set to a higher current limit for increased
charging and current availability at V_OUT. This mode is
typically used when there is power available from a wall
adapter.
Ideal Diode from BAT to V_OUT
The LTC3557/LTC3557-1 has an internal ideal diode as well
as a controller for an optional external ideal diode. Both
the internal and the external ideal diodes respond quickly
whenever V_OUT drops below BAT.
If the load increases beyond the input current limit, ad-
ditional current will be pulled from the battery via the
ideal diodes. Furthermore, if power to V_BUS (USB) or
V_OUT (external wall power or high voltage regulator) is
removed, then all of the application power will be provided
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