MAX8814
28V Linear Li+ Battery Charger
with Smart Autoboot Assistant
Maxim Integrated | 7www.maximintegrated.com
Charger Enable Input
The MAX8814 contains an active-low logic input (EN)
used to enable the charger. Drive EN low, leave uncon-
nected, or connect to GND to enable the charger-
control circuitry. Drive EN high to disable the charger-
control circuitry. EN has an internal 200kΩ pull-down
resistor.
POK Output
The open-drain POK output asserts low when V
IN
≥
4.25V and (V
IN
- V
BATT
) ≥ 40mV (typ, V
IN
rising). POK
requires an external pullup resistor (1MΩ typ) to an
external power supply. POK is high impedance when
V
BATT
≥ (V
IN
- 40mV).
Autobooting Assistant
The MAX8814 contains an autobooting assistant circuit
that generates an enable signal for system booting
(ABO). The booting assistant functions as an internal
“OR” gate (Figure 1). The first input is dependent on
the input voltage (V
IN
), and the second input is an
external signal applied to ABI. The first input (POK) is
driven high once V
IN
≥ 4.25V and (V
IN
- V
BATT
) ≥ 40mV
(typ, V
IN
rising).
The second input signal (ABI) is driven by an external
source. ABI enables an autoboot signal (ABO high)
when a battery is connected at BATT and is independent
of POK. If POK is pulled low, the booting assistant
always drives ABO high, regardless of ABI (see Table 1).
ABI is pulled to GND through an internal 200kΩ resistor.
If ABI is driven externally, a RC filter (R1 and C2 of
Figure 3) is required for ESD protection and noise filter-
ing. If ABI is supplied by a system’s internal GPIO, or
logic, the RC filter is not required.
Soft-Start
The soft-start algorithm activates when entering fast-
charge mode. In the MAX8814, when the prequalifica-
tion state is complete (V
BATT
≥ 2.5V), the charging
current ramps up in 250µs to the full charging current.
This reduces the inrush current on the input supply.
