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LTC4088-1
40881fa
capacitorareconnectedfromCLPROGtoGND,thevoltage
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).
TheaverageinputcurrentwillbelimitedbytheCLPROGpro-
grammingresistoraccordingtothefollowingexpression:
I
V
R
h
VBUS
BUSQ
cLPROG
=
+
(
)
1
where IBUSQ is the quiescent current of the LTC4088-1,
VCLPROG is the CLPROG servo voltage in current limit,
RCLPROG is the value of the programming resistor and
hCLPROG is the ratio of the measured current at VBUS to
the sample current delivered to CLPROG. Refer to the
Electrical Characteristics table for values of hCLPROG,
VCLPROG and IBUSQ. Given worst-case circuit tolerances,
the USB specification for the average input current in 1x
or 5x mode will not be violated, provided that RCLPROG is
2.94k or greater.
Table 1 shows the available settings for the D0, D1 and
D2 pins.
Table 1. Controlled Input Current Limit
D0
D1
D2
CHARGER
STATUS
IBUS(LIM)
0
Off
100mA (1x)
0
1
On
100mA (1x)
0
1
0
Off
500mA (5x)
0
1
On
500mA (5x)
1
0
Off
1A (10x)
1
0
1
On
1A (10x)
1
0
Off
2.5mA (Susp High)
1
Off
500A (Susp Low)
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 VOUT. 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 VOUT to approximately 300mV above
the voltage at BAT. However, if the voltage at BAT is below
3.3V,andtheloadrequirementdoesnotcausetheswitching
regulator to exceed its current limit, VOUT 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 VBUS, VOUT
will range between the no-load voltage and slighly below
the battery voltage, indicated by the shaded region of
Figure 2.
OPERATIO
U
BAT (V)
2.4
4.5
4.2
3.9
3.6
3.3
3.0
2.7
2.4
3.3
3.9
40881 F02
2.7
3.0
3.6
4.2
V OUT
(V)
NO LOAD
300mV
Figure 2. VOUT vs BAT
For very low-battery voltages, the battery charger acts like
a load and, due to limited input power, its current will tend
topullVOUTbelowthe3.6V“InstantOn”voltage.Toprevent
VOUT from falling below this level, an undervoltage circuit
automatically detects that VOUT is falling and reduces the
battery charge current as needed. This reduction ensures
that load current and voltage are always prioritized and yet
delivers as much battery charge current as possible. (See
Over Programming the Battery Charger in the Applications
Information section.)
The voltage regulation loop compensation is controlled by
the capacitance on VOUT. An MLCC capacitor of 10F is
required for loop stability. Additional capacitance beyond
this value will improve transient response.