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LTC4088EDE-2-TRPBF

High Efficiency Battery Charger/USB Power Manager with Regulated Output Voltage

Linear Technology 

LTC4088-1/LTC4088-2

Operation

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:

( ) IVBUS

=

IBUSQ

+

VCLPROG

RCLPROG

•

hCLPROG + 1

where IBUSQ is the quiescent current of the LTC4088-1/

LTC4088-2, 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

4088-1 4088-2 CHARGER

D0

D1

D2

D2 STATUS

0

0

0

1

Off

0

0

1

0

On

0

1

0

1

Off

0

1

1

0

On

1

0

0

1

Off

1

0

1

0

On

1

1

0

1

Off

1

1

1

0

Off

IBUS(LIM)

100mA (1x)

100mA (1x)

500mA (5x)

500mA (5x)

1A (10x)

1A (10x)

2.5mA (Susp High)

500µA (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, and the load requirement does not cause

the switching 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 slightly below the

battery voltage, indicated by the shaded region of Figure 2.

4.5

4.2

3.9

NO LOAD

3.6

3.3

300mV

3.0

2.7

2.4

2.4

2.7 3.0 3.3 3.6 3.9 4.2

BAT (V)

408812 F02

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

to pull VOUT below the 3.6V “Instant On” voltage. To prevent

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 10µF is

required for loop stability. Additional capacitance beyond

this value will improve transient response.

40881fc

11

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