MAX8588 Maxim Integrated Products, MAX8588 Datasheet - Page 28

• Manufacturer Part Number: MAX8588
• Description: High-Efficiency Low-IQ PMIC
• Manufacturer: Maxim Integrated Products

High-Efficiency, Low-I
Dynamic Core for PDAs and Smartphones
Figure 8. BKBT connection when no backup battery is used, or
if an alternate backup scheme, not involving the MAX8588, is
used.
Figure 9. A 1-cell rechargeable Li+ battery provides more back-
up power when a primary cell is insufficient. The cell is charged
to 3.3V when V1 is active. Alternately, the battery can be
charged from IN if the voltages are appropriate for the cell type.
Figure 10. A 1-cell NiMH battery can provide backup by boost-
ing with a low-power DC-DC converter. A series resistor-diode
trickle charges the battery when the main power is on.
28
BACKUP BATTERY
RECHARGEABLE
______________________________________________________________________________________
Li+ RECHARGEABLE
POWER
BACKUP BATTERY
MAIN
1-CELL
NiMH
4.7µF
1-CELL
4.7µF
1N4148
POWER
MAIN
4.7µF
1N4148
10kΩ
BATT
GND
4.7µF
MAX1724
LQH32C 10µH
1kΩ
EZK30
D1
1µF
1µF
MURATA
SHDN
OUT
10µF
LX
IN
V1
BKBT
V7
IN
BKBT
V7
4.7µF
MAIN
POWER
MAX8588
MAX8588
3.0V
1µF
IN
BKBT
V7
MAX8588
Q
PMIC with
If more backup power is needed and a primary cell has
inadequate capacity, a rechargeable lithium cell can be
accommodated as shown in Figure 9. A series resistor
and diode charge the cell when the 3.3V V1 supply is
active. In addition to biasing V7, the rechargeable bat-
tery may be required to also power other supplies.
In some systems, a NiMH battery may be desired for
backup. Usually this requires multiple cells because
the typical NiMH cell voltage is only 1.2V. By adding a
small DC-DC converter (MAX1724), the low-battery
voltage is boosted to 3V to bias BKBT (Figure 10). The
DC-DC converter’s low operating current (1.5µA typ)
allows it to remain on constantly so the 3V BKBT bias is
always present. A resistor and diode trickle charge the
NiMH cell when the main power is present.
Good PC board layout is important to achieve optimal
performance. Conductors carrying discontinuous cur-
rents and any high-current path should be made as
short and wide as possible. A separate low-noise
ground plane containing the reference and signal
grounds should connect to the power-ground plane at
only one point to minimize the effects of power-ground
currents. Typically, the ground planes are best joined
right at the IC.
Keep the voltage feedback network very close to the
IC, preferably within 0.2in (5mm) of the FB_ pin. Nodes
with high dV/dt (switching nodes) should be kept as
small as possible and should be routed away from
high-impedance nodes such as FB_.
Rechargeable NiMH Backup Battery
PC Board Layout and Routing
Rechargeable Li+ Backup Battery
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