MAX1586A_08 Datasheet, PDF (28/30 Pages) Maxim Integrated Products – High-Efficiency, Low-IQ PMICs with Dynamic Core for PDAs and Smart Phones

High-Efficiency, Low-I_Q PMICs with
Dynamic Core for PDAs and Smart Phones
MAIN
POWER
4.7μF
D1
1N4148
1μF
IN
MAX1586
BKBT ^MAX1587
V7
Figure 8. BKBT connection when no backup battery is used, or
if an alternate backup scheme, not involving the
MAX1586/MAX1587, is used.
MAIN
POWER
4.7μF
1kΩ
1-CELL
Li+ RECHARGEABLE
BACKUP BATTERY
4.7μF
1μF
IN
MAX1586
MAX1587
V1
BKBT
V7
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.
Rechargeable Li+ Backup Battery
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.
Rechargeable NiMH Backup Battery
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.
PCB Layout and Routing
Good PCB layout is important to achieve optimal perfor-
mance. Conductors carrying discontinuous currents and
any high-current path should be made as short and wide
as possible. A separate low-noise ground plane contain-
ing 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_. Refer to the
MAX1586 or MAX1587 evaluation kit data sheets for a
full PCB example.
1-CELL
NiMH
RECHARGEABLE
BACKUP BATTERY
1N4148
4.7μF
10kΩ
MURATA
LQH32C 10μH
BATT
LX
MAX1724
EZK30
SHDN
OUT
GND
10μF
MAIN
POWER
4.7μF
IN
MAX1586
MAX1587
3.0V
BKBT
V7
1μF
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 ______________________________________________________________________________________