1.2MHz, Low-Cost,

High-Performance Chargers

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13

The PWM filter accepts the digital signal with a frequency

from 128Hz to 500kHz. Zero duty cycle shuts down the

MAX17005B/MAX17006B/MAX17015B, and 99.5% duty

cycle corresponds to full scale (60mV) across CSIP

and CSIN.

Choose a current-sense resistor (RS2) to have a suffi-

cient power-dissipation rating to handle the full-charge

current. The current-sense voltage can be reduced to

minimize the power-dissipation period. However, this can

degrade accuracy due to the current-sense amplifier’s

input offset (0.25mV typ). See the

Typical Operating

Characteristics to estimate the charge-current accuracy

at various set points.

Setting Input-Current Limit

The total input current, from a wall adapter or other DC

source, is the sum of the system supply current and the

current required by the charger. When the input current

exceeds the set input-current limit, the controller

decreases the charge current to provide priority to sys-

tem load current. System current normally fluctuates as

portions of the system are powered up or down. The

input-current-limit circuit reduces the power require-

ment of the AC wall adapter, which reduces adapter

cost. As the system supply rises, the available charge

current drops linearly to zero. Thereafter, the total input

current can increase without limit.

The total input current is the sum of the device supply cur-

rent, the charger input current, and the system load cur-

rent. The total input current can be estimated as follows:

where

η is the efficiency of the DC-DC converter (typi-

cally 85% to 95%).

In the MAX17005B/MAX17006B/MAX17015B, the volt-

age across CSSP and CSSN is constant at 60mV.

Choose the current-sense resistor, RS1, to set the input

current limit. For example, for 4A input current limit,

choose RS1 = 15m

Ω. For the input current-limit set-

tings, which cannot be achievable with standard sense

resistor values, use a resistive voltage-divider between

CSSP and CSSN to tune the setting (Figure 4).

To minimize power dissipation, first choose RS1

according to the closest available value. For conve-

nience, choose Ra = 6k

Ω and calculate Rb from the

above equation.

Choose a current-sense resistor (RS1) to have a suffi-

cient power rating to handle the full system current. The

current-sense resistor can be reduced to improve effi-

ciency, but this degrades accuracy due to the current-

sense amplifier’s input offset (0.15mV typ). See the

Typical Operating Characteristics to estimate the input

current-limit accuracy at various set points.

Automatic Power-Source Selection

The MAX17005B/MAX17006B/MAX17015B use an

external charge pump to drive the gate of an n-channel

adapter selection switch (N3 and Q1a). In Figure 1,

when the adapter is present, BST is biased 5V above

As long as the adapter is present, even though the

charger is off, the power stage forces a refresh pulse to

the BST charge pump every 5ms.

When the adapter voltage is removed, the charger

stops generating BST refresh pulses and N4 forces N2

off, Q1b turns on and supplies power to the system

from the battery.

In Figure 1, D1 must have low forward-voltage drop and

low reverse-leakage current to ensure sufficient gate

drive at N3 and Q1a. A 100mA, low reverse-leakage

Schottky diode is the right choice.

Analog Input Current-Monitor Output

Use IINP to monitor the system-input current, which is

sensed across CSSP and CSSN. The voltage at IINP is

proportional to the input current:

nected between IINP and ground. Typically, IINP has a

0 to 3.5V output-voltage range. Leave IINP unconnected

when not used.

I

V

RS

G

R

INPUT

IINP

IINP

IINP

=

××

1

I

mV

RS

Rb

Ra

INPUT LIMIT

_

()

=× +

60

1

1

II

IV

V

INPUT

LOAD

CHARGE

BATTERY

IN

=+

×

×η

CSSP

CSSN

RS1

Ra

Rb

MAX17005B/MAX17006B/MAX17015B

Figure 4. Input Current-Limit Fine Tuning