MAX8568A Просмотр технического описания (PDF) - Maxim Integrated

Номер в каталоге

Компоненты Описание

производитель

MAX8568A

Complete Backup-Management ICs for Lithium and NiMH Batteries

Maxim Integrated 

Complete Backup-Management ICs

for Lithium and NiMH Batteries

Detailed Description

The MAX8568A/MAX8568B are compact ICs for manag-

ing backup battery charging and utilization in PDAs and

other smart handheld devices. The MAX8568A/

MAX8568B are comprised of three major blocks: 1) A

multichemistry charger for small lithium-ion, lithium-man-

ganese, LiVeO5, and NiMH batteries; 2) a small very-

low-current step-up DC-DC converter that generates a

boosted backup supply when the backup battery output

is less than required; and 3) an LDO that supplies a

second backup voltage to an additional system block

(typically low-voltage RAM).

Multichemistry Charger

The backup battery charger charges most types of

rechargeable lithium and NiMH cells. Charging current

can be set up to 25mA by a resistor connected from

CHGI to GND. The charger operates a current-limited

voltage source for rechargeable lithium batteries, and

switches between fast and trickle charging for NiMH

batteries.

NiMH Charging Scheme

The NiMH charger operates at two different charge cur-

rents based upon the voltages at TERMV and STRTV.

VSTRTV sets the BK voltage below which fast charging

(set by CHGI) occurs. VTERMV sets the upper BK trip

point where fast charging stops and trickle charging

begins, and also sets a maximum voltage limit for the

NiMH battery. If VTERMV is 1.2V, then fast charge stops

at 1.2 / 0.86 = 1.4V, and the maximum voltage limit is

1.2 / 0.67 = 1.791V.

An NiMH battery fast charges until it hits 1.4V set by

VTERMV. The charger then switches to trickle charge at

a current that is 10% of fast charge (set by CHGI). If the

voltage drops (due to loading or self-discharge) to 1.2V

(with VSTRTV = 1.2V), fast charge resumes. If the volt-

age then increases back to 1.4V (with VTERMV = 1.2V),

trickle charge resumes. If the cell voltage reaches 1.8V,

the charge current falls to zero.

Lithium Charging Scheme

When charging rechargeable lithium-type batteries,

VTERMV sets the charging voltage while VSTRTV is unused.

Charge current is set by a resistor from CHGI to GND.

There is no trickle charge for lithium mode. This charging

scheme is essentially a current-limited voltage source.

Step-Up DC-DC Converter

If an NiMH battery or lower-voltage rechargeable lithium

battery is used for backup, it may be necessary to boost

the battery voltage to 2.5V, 3.3V, or some other voltage to

power RAM, RTC, or other devices. The step-up DC-DC

converter is powered by the backup battery but requires

that the I/O supply be activated at least one time before

the backup battery can be stepped up. This allows the

end product to draw no backup battery current while “on

the shelf” waiting for its first activation. The step-up DC-

DC converter is enabled, and reaches regulation, 50µs

(typ) after INOK falls below 2.43V (typ).

The step-up converter includes a built-in synchronous

rectifier that reduces cost by eliminating the need for

an external diode and improves overall efficiency. The

converter also features a clamp circuit that reduces

EMI due to inductor ringing. The output voltage is set to

3.3V or 2.5V by connecting BKV to either GND or

BKSU, respectively. For adjustable output, connect

BKV to a resistor-divider from BKSU to GND.

LDO

For designs that require two different backup voltages,

the MAX8568 includes a small LDO that is powered from

BKSU. This LDO can supply up to 10mA and uses only

5µA of operating current. The LDO output is preset to

2.5V in the MAX8568A and 1.8V in the MAX8568B. The

LDO is activated after VINOK falls below 2.43V (typ).

Switchover Behavior

See Figure 1 for switchover timing. If the backup bat-

tery is connected to the system before main power, the

MAX8568 remains off and draws very little current, typi-

cally less than 0.5µA. This allows the end product to

draw no backup battery current while “on the shelf”

waiting for its first activation. When main power is con-

nected, the MAX8568 powers on, assuming the main

battery is greater than 2.8V. The MAX8568 begins to

charge the backup battery if needed (see the

Multichemistry Charger section). The OD1 and OD2

outputs pull to GND and turn on the external p-channel

MOSFETs. This allows the voltage on I/O IN and MEM

IN (Figure 7) to pass through to the I/O OUT and MEM

OUT outputs. These I/O and MEM voltages are typically

provided by a MAX1586/MAX1587 power-supply IC.

INOK monitors the main battery voltage and activates

the backup boost converter and LDO when the voltage

on VINOK falls below 2.43V. The backup converter

starts 50µs after VINOK falls. OD1 and OD2 go high

impedance and turn off the external p-channel

MOSFETs. These MOSFETs disconnect the I/O IN and

MEM IN inputs from the load. This ensures that the I/O

and MEM main supplies do not draw current from the

backup source (MAX8568). The charger also turns off

when INOK is less than 2.43V.

If the MAX8568 is being evaluated as a stand-alone

device, note that the backup-battery boost converter will

not operate unless I/O IN has been activated at least one

time. The typical power removal sequence for testing is 1)

main battery goes low, then 2) MEM IN and I/O IN go low.

_______________________________________________________________________________________ 9

Share Link: