LTC3555 Linear Technology, LTC3555 Datasheet - Page 15

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

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If the combined load at V
switching power supply to reach the programmed input
current limit, the battery charger will reduce its charge cur-
rent by that amount necessary to enable the external load
to be satisfi ed. Even if the battery charge current is set to
exceed the allowable USB current, the USB specifi cation
will not be violated. The switching regulator will limit the
average input current so that the USB specifi cation is never
violated. Furthermore, load current at V
prioritized and only excess available power will be used
to charge the battery.
If the voltage at BAT is below 3.3V, or the battery is not
present, and the load requirement does not cause the
switching regulator to exceed the USB specifi cation,
V
able power, V
the battery voltage. If there is no battery present when
the load exceeds the available USB power, V
toward ground.
The power delivered from V
by a 2.25MHz constant-frequency step-down switching
regulator. To meet the USB maximum load specifi cation,
the switching regulator includes a control loop which
ensures that the average input current is below the level
programmed at CLPROG.
The current at CLPROG is a fraction (h
current. When a programming resistor and an averaging
capacitor are connected from CLPROG to GND, the voltage
on CLPROG represents the average input current of the
switching regulator. When the input current approaches
the programmed limit, CLPROG reaches V
and power out is held constant. The input current limit
is programmed by the I
serial port. 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 according to the following expression:
Figure 1 shows the range of possible voltages at V
a function of battery voltage.
OPERATION
OUT
I
VBUS
will regulate at 3.6V. If the load exceeds the avail-
=
I
VBUSQ
OUT
CLPROG
will drop to a voltage between 3.6V and
+
R
V
CLPROG
CLPROG
servo voltage and the resistor on
LIM0
OUT
is large enough to cause the
and I
BUS
•
(
h
LIM1
CLPROG
to V
CLPROG
pins or by the I
OUT
OUT
CLPROG
+
will always be
–1
1
OUT
is controlled
)
) of the V
can drop
, 1.188V,
OUT
BUS
2
as
C
Ideal Diode from BAT to V
The LTC3555 has an internal ideal diode as well as a con-
troller for an optional external ideal diode. The ideal diode
controller is always on and will respond quickly whenever
V
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
of the application power will be provided by the battery
via the ideal diode. The transition from input power to
battery power at V
the 10μF capacitor to keep V
diode consists of a precision amplifi er that enables a large
on-chip P-channel MOSFET transistor whenever the voltage
OUT
drops below BAT.
1000
2200
2000
1800
1600
1400
1200
800
600
400
200
4.5
4.2
3.9
3.6
3.3
3.0
2.7
2.4
0
2.4
BUS
0
Figure 2. Ideal Diode Operation
FORWARD VOLTAGE (mV) (BAT – V
(USB or wall power) is removed, then all
60
2.7
IDEAL DIODE
NO LOAD
OUT
Figure 1. V
120
VISHAY Si2333
OPTIONAL EXTERNAL
IDEAL DIODE
LTC3555
SEMICONDUCTOR
3.0
will be quick enough to allow only
MBRM120LT3
180
BAT (V)
OUT
3.3
240 300
OUT
OUT
ON
vs BAT
3.6
from drooping. The ideal
300mV
360
3.9
420
OUT
LTC3555
3555 F01
3555 F02
)
480
4.2
15
3555f