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

MAX17005/MAX17006/MAX17015, 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 amplifi-

er’s input offset (0.25mV typ). See

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-to-DC converter

(typically 85% to 95%).

In the MAX17005/MAX17006/MAX17015, the voltage

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 settings, which

cannot be achievable with standard sense resistor val-

ues, 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

Typical

Operating Characteristics to estimate the input current-

limit accuracy at various set points.

Automatic Power-Source Selection

The MAX17005/MAX17006/MAX17015 use an external

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

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

so that N3 and Q1a are on, and Q1b is off. 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

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

ed 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

MAX17005/MAX17006/MAX17015

Figure 4. Input Current-Limit Fine Tuning