MAX8568B Просмотр технического описания (PDF) - Maxim Integrated
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MAX8568B Datasheet PDF : 17 Pages
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Complete Backup-Management ICs
for Lithium and NiMH Batteries
Note that both VBK(NILO) and VBK(NIHI) can be set with a
2-resistor voltage-divider as shown in the typical applica-
tion circuit (see Figure 7) if the factory-set ratio between
the two thresholds is acceptable. In that case:
R6
=
R8
VREF
VBK(NILO)
− 1
VBK(NIHI) = 1.163 x VBK(NILO)
VBK(NIMAX) = 1.283 x VBK(NIHI)
One 3-resistor-divider can be used to set both
VBK(NILO) and VBK(NIHI) independently. Figure 4 shows
the connections of R17, R18, and R19. Select R19 in
the 100kΩ to 1MΩ range. The equations for the two
upper divider-resistors are:
R18
=
R19
VREF
VBK(NILO)
− 1
R17
=
(R18
+ R19)
×
1.163 × VREF
VBK(NIHI)
− 1
Setting the Switchover Voltage
VINOK sets the IN voltage at which backup mode starts.
INOK connects to a resistor-divider between IN and
GND. The MAX8568 requires VIN greater than 2.8V for
proper operation when not backing up, so the backup
threshold, VIN(BACKUP), must be set for no less than
2.8V. Once VINOK drops below 2.43V (typ), VIN may be
less than 2.8V. The resistor-divider for INOK is shown in
Figure 7 (R9 and R10). Select resistor R10 to be in the
100kΩ to 1MΩ range. Calculate R9 as follows:
R9 =
R10
VIN(BACKUP)
VINOK
− 1
where VINOK = 2.43V, and VIN(BACKUP) must be set
greater than 2.8V.
Step-Up Converter
The step up DC-DC converter is most likely used with
NiMH backup batteries, but can also be used with
rechargeable lithium backup batteries. If the backup
battery voltage is greater than the set output voltage at
BKSU, the output voltage follows the backup battery
voltage. The voltage difference between the backup
battery and BKSU never exceeds a diode forward-volt-
age drop. If I/O OUT (Figure 7) is less than BK during
charge mode, no current flows from BK to I/O OUT.
16
REF
R17
15
TERMV
R18
14
STRTV
R19
Figure 4. 3-Resistor Divider Used to Set VBK(NILO) and VBK(NIHI)
Step-Up Capacitor Selection
Choose output capacitors to supply output peak cur-
rents with acceptable voltage ripple. Low equivalent-
series-resistance (ESR) capacitors are recommended.
Ceramic capacitors have the lowest ESR, but low-ESR
tantalum or polymer capacitors offer a good balance
between cost and performance.
Output voltage ripple has two components: variations in
the charge stored in the output capacitor with each LX
pulse and the voltage drop across the capacitor’s ESR
caused by the current into and out of the capacitor. The
equations for calculating output ripple are:
VRIPPLE = VRIPPLE(C) + VRIPPLE(ESR)
VRIPPLE(ESR) = IPEAK x RESR(CBKSU)
( ) VRIPPLE(C)
=
1
2
VBKSU
L
− VBK
CBKSU
IPEAK2
where IPEAK is the peak inductor current (see the
Inductor Selection section). For ceramic capacitors, the
output voltage ripple is typically dominated by
VRIPPLE(C).
Input capacitors connected to IN and BK should be
X5R or X7R ceramic capacitors. CIN should be 4.7µF or
greater. CBK should be 10µF or greater when using the
step-up converter. If the step-up converter is not used,
then CBK can be reduced to 1µF.
Capacitance and ESR variation with temperature should
be considered for best performance in applications with
wide operating temperature ranges.
12 ______________________________________________________________________________________
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