Datasheet

LTC4088
11
4088fb
operaTion
from V
BUS
, load current will be drawn from the battery via
the ideal diodes even when the battery charger is enabled.
The current at CLPROG is a precise fraction of the V
BUS
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. As the input current approaches the
programmed limit, CLPROG reaches 1.188V and power
delivered by the switching regulator is held constant.
Several ratios of current are available which can be set
to correspond to USB low and high power modes with a
single programming resistor.
The input current limit is programmed by various com-
binations of the D0, D1 and D2 pins as shown in Table 1.
The switching input regulator can also be deactivated
(USB Suspend).
The average input current will be limited by the CLPROG
programming resistor according to the following expres-
sion:
I
VBUS
= I
BUSQ
+
V
CLPROG
R
CLPROG
• h
CLPROG
+ 1
( )
where I
BUSQ
is the quiescent current of the LTC4088,
V
CLPROG
is the CLPROG servo voltage in current limit,
R
CLPROG
is the value of the programming resistor and
h
CLPROG
is the ratio of the measured current at V
BUS
to
the sample current delivered to CLPROG. Refer to the
Electrical Characteristics table for values of h
CLPROG
,
V
CLPROG
and I
BUSQ
. Given worst-case circuit tolerances,
the USB specification for the average input current of 1x
or 5x mode will not be violated, provided that R
CLPROG
is
2.94k or greater.
Table 1 shows the available settings for the D0, D1 and
D2 pins.
Notice that when D0 is high and D1 is low, the switching
regulator is set to a higher current limit for increased
charging and power availability at V
OUT
. These modes
will typically be used when there is line power available
from a wall adapter.
While not in current limit, the switching regulator’s
Bat-Track feature will set V
OUT
to approximately 300mV
above the voltage at BAT. However, if the voltage at BAT
is below 3.3V, and the load requirement does not cause
the switching regulator to exceed its current limit, V
OUT
will regulate at a fixed 3.6V as shown in Figure 2. This
will
allow a portable product to run immediately when power
is applied without waiting for the battery to charge.
If the load does exceed the current limit at V
BUS
, V
OUT
will
range between the no-load voltage and slightly below the
battery voltage, indicated by the shaded region of Figure 2.
If there is no battery present when this happens, V
OUT
may
collapse to ground.
The voltage regulation loop compensation is controlled by
the capacitance on V
OUT
. An MLCC capacitor of 10µF is
required for loop stability. Additional capacitance beyond
this value will improve transient response.
Table 1. Controlled Input Current Limit
D0 D1 D2
CHARGER
STATUS I
BUS(LIM)
0 0 0 On 100mA (1x)
0 0 1 Off 100mA (1x)
0 1 0 On 500mA (5x)
0 1 1 Off 500mA (5x)
1 0 0 On 1A (10x)
1 0 1 Off 1A (10x)
1 1 0 Off 500µA (Susp Low)
1 1 1 Off 2.5mA (Susp High)
Figure 2. V
OUT
vs BAT
BAT (V)
2.4
4.5
4.2
3.9
3.6
3.3
3.0
2.7
2.4
3.3 3.9
4088 F02
2.7 3.0
3.6 4.2
V
OUT
(V)
NO LOAD
300mV