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

Inductor Selection

The control scheme of the MAX8568 permits flexibility in

choosing an inductor. A 10µH inductor performs well in

most applications. Smaller inductance values typically

offer smaller physical size for a given series resistance,

allowing the smallest overall circuit dimensions. Circuits

using larger inductance may provide higher efficiency

and exhibit less ripple, but also may reduce the maxi-

mum output current. This occurs when the inductance is

sufficiently large to prevent the LX current limit (ILIM)

from being reached before the maximum on-time

(tON(MAX)) expires.

For maximum output current, choose the inductor value

so that the controller reaches the current limit before

the maximum on-time is reached:

L < VBK × tON(MAX)

ILIM

where tON(MAX) is typically 5µs, and the current limit (ILIM)

is typically 500mA (see the Electrical Characteristics

table).

For larger inductor values, determine the peak inductor

current (IPEAK) by:

IPEAK

=

VBK × tON(MAX)

L

Setting the Output Voltage

The output voltage is set to 2.5V or 3.3V, or is

adjustable. Connect BKV to GND for 3.3V, and BKV to

BKSU for 2.5V. The adjustable output voltage is set

from 2.5V to 5V using external resistors R1 and R2

(Figure 7). Since FB leakage is 50nA (max), select

feedback resistor R2 in the 100kΩ to 1MΩ range.

Calculate R1 as follows:

R1

=

R2





VBKSU

VBKV



− 1

where VBKV = 1.21V.

LDO

The LDO output voltage is preset to 2.5V for the

MAX8568A and 1.8V for the MAX8568B. The LDO can

supply up to 10mA. The LDO output voltage is not

adjustable.

LDO Capacitor Selection

Capacitors are required at the LDO output of the

MAX8568 for stable operation over the full load and tem-

perature range. A 4.7µF or greater X5R or X7R ceramic

capacitor is recommended. To reduce noise and

improve load-transient response, larger output capaci-

tors up to 10µF can be used. Surface-mount ceramic

capacitors have very low ESR and are commonly avail-

able in values up to 10µF. Note that some ceramic

dielectrics, such as Z5U and Y5V, exhibit large capaci-

tance and ESR variation with temperature and require

larger than the recommended values to maintain stability

and good load-transient response over temperature.

External MOSFET Drivers—OD1, OD2

OD1 and OD2 are open-drain outputs and are

designed to be connected to the gates of external p-

channel MOSFETs (see Figure 7). These MOSFETs

connect the main system power supplies (I/O IN and

MEM IN) to the system loads (I/O OUT and MEM OUT)

during normal operation. During backup, they discon-

nect the power supplies from the system loads to pre-

vent the power supplies from drawing backup current

away from the system. For this reason, the MOSFETs

are connected “backwards” from what might be

expected. The source of the MOSFETs are connected

to the system load side (I/O OUT and MEM OUT). The

MOSFETs’ purpose is to block current flow from the

backup supply (BKSU) to the main supplies (I/O IN and

MEM IN). They do not block current flow from I/O IN to

I/O OUT and from MEM IN to MEM OUT. Even when off,

the MOSFET body diodes allow current to pass in that

direction.

OD1 is intended to drive the MOSFET switch for I/O IN

and I/O OUT, while OD2 is intended to drive the MOSFET

switch for MEM IN and MEM OUT. See the Typical

Operating Characteristics and Figure 1 for typical opera-

tion of OD1 and OD2.

External MOSFET Selection

The external MOSFET should be chosen based upon

RDS(ON) and gate capacitance. When VINOK > 2.43V

(main battery > 2.8V), the current required for normal

operation of I/O and MEM goes through these external

MOSFETs. Choose an RDS(ON) that minimizes the

MOSFET voltage drop. When VINOK < 2.43V, the

MOSFET turns off, and MEM and I/O are powered by

the MAX8568. The gate capacitance of the external

MOSFET must discharge through the external gate-to-

source resistor. This discharge time determines how

quickly the main supply is disconnected and isolated.

______________________________________________________________________________________ 13

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