MAX8568A 查看數據表（PDF） - Maxim Integrated

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MAX8568A

Complete Backup-Management ICs for Lithium and NiMH Batteries

Maxim Integrated 

Complete Backup-Management ICs

for Lithium and NiMH Batteries

Pullup resistors, R3 and R4 in Figure 7, should be select-

ed to ensure that when OD1 and OD2 go high imped-

ance, the gate of the external MOSFET discharges within

50µs to 100µs. This time allows the backup converters to

start and provide power to I/O and MEM. Discharges

longer than 50µs to 100µs could cause the main supply

to back drain current from the MAX8568 and allow the

I/O OUT and MEM OUT voltage to droop. The MOSFET

gate-source resistor, RGS, is calculated from the follow-

ing formulas:

τ = RGS x CISS

τ=

−50µs



ln 1 −

VGS(TH) 

VBKSU 

where the MOSFET gate-source threshold, VGS(TH),

and MOSFET input capacitance, CISS, are provided on

the MOSFET data sheet.

Connection with MAX1586

When the MAX8568 is used with the MAX1586 system

power supply, it may be preferable to employ the

MAX1586’s voltage monitors to determine when backup

should start. The connection for this is shown in Figure 5

where the dead-battery output (DBO) of the MAX1586

drives the INOK input of the MAX8568. This, in effect,

overrides the voltage-sensing circuit on the MAX8568

and uses the DBO monitor on the MAX1586. Refer to the

MAX1586 data sheet for information on how to set the

DBO threshold. The CHG connection in Figure 5 is

described in the next section.

Terminating Charging at a Voltage Other

than the Switchover Voltage

In normal operation, the MAX8568 charger is always

active as long as the INOK voltage is valid (above

2.43V). In some systems, however, it may be desirable to

terminate backup battery charging when the main bat-

tery is somewhat depleted but not so low as to trigger

backup. An external voltage monitor, or a voltage moni-

tor in a power-supply IC, such as the MAX1586, can dis-

able charging by disconnecting the CHGI resistor. If

CHGI is open, no charging current flows. This can be

accomplished with the circuit in Figure 5. The low-battery

output (LBO) of the MAX1586 pulls low when the battery

falls below a user-set level (refer to the MAX1586 data

sheet). This turns off the external n-channel MOSFET (or

IN

MAX1586

LBO

DBO

CHGI

1MΩ

RCHGI

n-CHANNEL

MOSFET OR

OPEN-DRAIN

INVERTER

MAX8568

1MΩ

INOK

Figure 5. Using a MAX1586 Power-Supply IC to Trigger

Backup Switchover and to Disable Backup Battery Charging

Prior to Switchover

open-drain logic inverter) and disconnects the current

path through RICHG. Backup charging can be stopped

for any reason using this method.

PC Board Layout and Routing

Careful PC board layout is important for minimizing

ground bounce and noise. Ensure that C1 (IN input

capacitor), C2 (BK input capacitor), C3 (BKSU bypass

capacitor), and C4 (LDO output capacitor) are as close

as possible to the IC. Avoid using vias to connect C2 or

C3 to their respective pins or GND. C2 and C3 grounds

should be next to each other, and this connection can

then be used as the star ground point. All other grounds

should connect to the star ground. PGND should star at

C2 and C3, and should not connect directly to the

exposed pad (EP) of the MAX8568. Connect EP to the

bottom layer ground plane, and then connect the

ground plane to the star ground. Vias on the inductor

path are acceptable if necessary. IN, BK, BKSU, and

LDO traces should be as wide as possible to minimize

inductance. Refer to the MAX8568 evaluation kit for a

PC board layout example.

Chip Information

TRANSISTOR COUNT: 7902

PROCESS: BiCMOS

14 ______________________________________________________________________________________

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