MAX4978–MAX4981
Detailed Description
The MAX4978–MAX4981 family of devices is a combi-
nation of an overvoltage protector, overcurrent limiter,
and lithium-ion battery overcharge protector (MAX4980/
MAX4981). These devices feature a low 85mΩ (typ)
R
ON
FET and are used to protect the charger input port
on a portable device. If the input voltage exceeds the
overvoltage threshold (OVLO) or falls below the under-
voltage threshold (UVLO), the FET is turned off to pre-
vent damage to the protected components. The internal
charge pump’s 160ms debounce time prevents false
turn-on of the internal FET during startup. SDT is an
active-low open-drain output that asserts low during the
debounce time after a valid voltage is applied to the
input (MAX4978/MAX4979).
The MAX4978–MAX4981 feature an overcurrent limiter
as a second layer of protection for a lithium-ion battery
charger that limits current to a 900mA (MAX4978/
MAX4979/MAX4980) or 1950mA (MAX4981) threshold
for a 20ms blanking time. At the end of the blanking time,
if current is still at the threshold, the FET is turned off and
the device restarts the cycle after 160ms. The battery
voltage monitor measures the voltage of a lithium-ion
battery and disables the FET if the battery voltage
exceeds 4.4V (MAX4980/MAX4981).
Autoretry
When the current-limit threshold is reached, the t
BLANK
timer begins counting. The timer resets if the overcur-
rent condition disappears before t
BLANK
has elapsed.
A retry time delay, t
RETRY
, is started immediately after
t
BLANK
has elapsed and during that time, the FET is off.
At the end of t
RETRY
, the FET is turned on again. If the
fault still exists, the cycle is repeated. If the fault has
been removed, the FET stays on.
Undervoltage Lockout (UVLO)
The MAX4978 has a 4.4V UVLO threshold, while the
MAX4979/MAX4980/MAX4981 have a 2.63V UVLO
threshold. When an undervoltage lockout condition
occurs with high load current, the voltage at the input to
the UVLO comparator may recover due to internal para-
sitic resistance causing the device to restart.
Overvoltage Lockout (OVLO)
The MAX4979 has a 6.8V OVLO threshold, while the
MAX4978/MAX4980/MAX4981 have a 5.7V OVLO
threshold.
Battery Overcharge Protector
(MAX4980/MAX4981)
The MAX4980/MAX4981 feature a battery overcharge
protection input (BAT) that triggers a voltage fault, turn-
ing off the FET, when V
BAT
exceeds 4.4V.
Startup Debounce Time Indicator (
SDT
)
(MAX4978/MAX4979)
SDT is an open-drain output that asserts low during the
startup debounce time and during the autoretry time.
SDT returns to high impedance once the charge pump
turns on. This feature is useful for discharging the AC
adapter capacitance during the startup debounce time
(Figures 1, 2).
Thermal Shutdown Protection
The MAX4978–MAX4981 have a thermal-shutdown fea-
ture to protect the devices from overheating. The
device immediately turns off when the junction temper-
ature exceeds +160°C (typ). These devices exit thermal
shutdown after the junction temperature cools by 10°C
(typ).
EN
Input
EN is an active-low enable input. Drive EN low for nor-
mal operation. Drive EN high to disable the device.
Applications Information
IN Bypass Capacitor
For most applications, bypass IN to GND with a 1µF
ceramic capacitor as close to the device as possible to
enable ±15kV (HBM) ESD protection on IN. If the power
source has significant inductance due to long lead
length, take care to prevent overshoots due to the LC
tank circuit and provide protection if necessary to pre-
vent exceeding the 30V absolute maximum rating on IN.
BAT Bypass Capacitor
For most applications, bypass BAT to GND with a 1µF
ceramic capacitor as close to the device as possible to
enable ±15kV (HBM) ESD protection on BAT. If high
ESD protection is not necessary, no capacitor is
required on BAT.
ESD Test Conditions
ESD performance depends on a number of conditions.
The MAX4978–MAX4981 are specified for ±15kV
(HBM) typical ESD resistance on IN and BAT when IN
and BAT are bypassed to ground with a 1µF low ESR
ceramic capacitor. Contact Maxim for a reliability report
that documents test setup, methodology, and results.
Overvoltage Protector with
Active Current Limit
8 _______________________________________________________________________________________
