MAX8730 Просмотр технического описания (PDF) - Maxim Integrated

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MAX8730

Low-Cost Battery Charger

Maxim Integrated 

Low-Cost Battery Charger

Detailed Description

The MAX8730 includes all the functions necessary to

charge Li+, NiMH, and NiCd batteries. A high-efficien-

cy, step-down, DC-DC converter is used to implement

a precision constant-current, constant-voltage charger.

The DC-DC converter drives a p-channel MOSFET and

uses an external free-wheeling Schottky diode. The

charge current and input current-sense amplifiers have

low-input offset errors, allowing the use of small-value

sense resistors for reduced power dissipation. Figure 2

is the functional diagram.

The MAX8730 features a voltage-regulation loop (CCV)

and two current-regulation loops (CCI and CCS). The

loops operate independently of each other. The CCV

voltage-regulation loop monitors BATT to ensure that its

voltage never exceeds the voltage set by VCTL. The

CCI battery current-regulation loop monitors current

delivered to BATT to ensure that it never exceeds the

current limit set by ICTL. The charge-current-regulation

loop is in control as long as the battery voltage is below

the set point. When the battery voltage reaches its set

point, the voltage-regulation loop takes control and

maintains the battery voltage at the set point. A third

loop (CCS) takes control and reduces the charge cur-

rent when the adapter current exceeds the input cur-

rent limit set by CLS.

The ICTL, VCTL, and CLS analog inputs set the charge

current, charge voltage, and input-current limit, respec-

tively. For standard applications, default set points for

VCTL provide 4.2V per-cell charge voltage. The MODE

input selects a 3- or 4-cell mode.

Based on the presence or absence of the AC adapter,

the MAX8730 provides an open-drain logic output sig-

nal (A C O K) and connects the appropriate source to the

system. P-channel MOSFETs controlled from the PDL

and PDS select the appropriate power source. The

MODE input allows the system to perform a battery

relearning cycle. During a relearning cycle, the battery

is isolated from the charger and completely discharged

through the system load. When the battery reaches

100% depth of discharge, PDL turns off and PDS turns

on to connect the adapter to the system and to allow the

battery to be recharged to full capacity.

Setting Charge Voltage

The VCTL input adjusts the battery output voltage, VBATT.

This voltage is calculated by the following equation:

VBATT

= CELLS x (4V +

VVCTL )

9

where CELLS is the number of cells selected with the

MODE input (see Table 1). Connect MODE to LDO for 4-

cell operation. Float the MODE input for 3-cell operation.

The battery-voltage accuracy depends on the absolute

value of VCTL, and the accuracy of the resistive volt-

age-divider that sets VCTL. Calculate the battery volt-

age accuracy according to the following equation:

VBATT _ ERROR

=

E0

+

100%

x





IVCTL

x RVCTL

36

− 1

where E0 is the worst-case MAX8730 battery voltage

error when using 1% resistors (0.83%), IVCTL is the

VCTL input bias current (4µA), and RVCTL is the imped-

ance at VCTL. Connect VCTL to LDO for the default

setting of 4.20V/cell with 0.7% accuracy.

Connect MODE to GND to enter relearn mode, which

allows the battery to discharge into the system while

the adapter is present; see the Relearn Mode Section.

Table 1. Cell-Count Programming

CELLS

GND

Float

LDO

CELL COUNT

Relearn mode

3

4

Setting Charge Current

ICTL sets the maximum voltage across current-sense

resistor RS2, which determines the charge current. The

full-scale differential voltage between CSIP and CSIN is

135mV (4.5A for RS2 = 30mΩ). Set ICTL according to

the following equation:

VICTL

=

ICHG

x

RS2

x

3.6V

135mV

The input range for ICTL is 0 to 3.6V. To shut down the

charger, pull ICTL below 65mV. Choose a current-sense

resistor (RS2) to have a sufficient power rating to handle

the full-charge current. The current-sense voltage may

be reduced to minimize the power dissipation. However,

this can degrade accuracy due to the current-sense

amplifier’s input offset (±2mV). See the Typical

Operating Characteristics to estimate the charge-cur-

rent accuracy at various set points. The charge-current

error amplifier (GMI) is compensated at the CCI pin.

See the Compensation section.

16 ______________________________________________________________________________________

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