Rainbow Electronics MAX4880 - Manual (Page 7)

Detailed Description
The MAX4880 provides up to 28V overvoltage protection
for low-voltage systems. When the input voltage at IN
exceeds the overvoltage trip level (OVLO), the MAX4880
turns off a low-cost, external n-channel MOSFET to pre-
vent damage to the protected components and issues
an overvoltage fault flag.
When the correct adapter is plugged in, the n-channel
MOSFET is turned on. The output of the MOSFET is
then connected to the internal current-limit switch that
provides the charge-current path to the battery. When
the battery reaches the trip voltage (4.2V), the internal
switch turns off and
BAT_OK asserts low, indicating
that the battery has reached its full charged state. The
internal switch turns back on only when the battery volt-
age drops by more than 200mV.
IN Overvoltage Lockout (OVLO)
The MAX4880 has a 5.6V typical overvoltage threshold
(OVLO). When V
IN is higher than V OVLO , GATE goes
low to turn off the external n-channel MOSFET. An over-
voltage
FLAGV is asserted low to notify the processor
of the fault condition.
IN Undervoltage Lockout (UVLO)
The MAX4880 includes a fixed 4.35V typical undervolt-
age-lockout level (UVLO). When V
IN is below the V UVLO
(1.2V ≤ V
IN
≤ 4.35V), GATE goes low to turn off the
external n-channel MOSFET. In addition, the driver for
the internal switch (BTA-BTB) is also turned off; there-
fore, this switch is open. This ensures the reverse cur-
rent, drained from the battery, is less than 1µA when
the adapter is not present.
Fault Flag Output (
FLAGV
)
The FLAGV output signals the host system that there is
a fault with the input voltage. FLAGV asserts low in
response to either an overvoltage or an undervoltage
fault.
FLAGV stays low for 50ms after GATE turns on,
before deasserting high.
FLAGV is an open-drain, active-low output. Connect a
pullup resistor from
FLAGV to the logic I/O voltage of
the host system or to any voltage source up to 6V.
FLAGV is invalid when driving EN high.
Battery-Voltage-Limit Flag Output
(
BAT_OK
)
The MAX4880 includes a battery-voltage-limit flag out-
put (
BAT_OK). BAT_OK asserts low to indicate the volt-
age on BTB exceeds the BTB trip level of 4.2V.
BAT_OK deasserts high when the voltage on BTB falls
by the BTB hysteresis voltage of more than 200mV.
BAT_OK is an open-drain, active-low output. Connect a
pullup resistor from
BAT_OK to the logic I/O voltage of
the host system, or to any voltage source up to 6V.
BAT_OK is invalid when driving EN high.
EN
Input
The MAX4880 features an active-low enable input ( EN).
Drive EN low or connect to ground for normal opera-
tion. Drive EN high to force the external n-channel
MOSFET off, disabling FLAGV and BAT_OK.
Internal Current Limit (BTA to BTB)
The internal switch from BTA to BTB has a preset cur-
rent-limit of 525mA (typ). If the load current from BTA to
BTB reaches this current limit, the switch operates in
the continuous mode, limiting the load current to the
preset value.
The switch remains in the current-limit condition until the
battery voltage on BTB exceeds 4.2V, or until the control
bit CB is driven low to open the switch.
Internal Switch Control Input (CB)
The CB input controls the internal switch. When CB is
high, the on/off state of the internal switch depends on
the battery voltage level. The internal switch turns off
when the battery voltage reaches the BTB trip level,
MAX4880
Overvoltage-Protection Controller with
Internal Disconnect Switch
_______________________________________________________________________________________ 7
IN
OVLO
UVLO
10V
CHARGE PUMP
GATE
IN
BAT_OK
FLAGV
EN
CHARGE
PUMP
GND
525mA
LIMIT
I REF
V REF
BTA
UVLO
BTA
BTB
BTB_SNS
CB
MAX4880
Figure 5. Functional Diagram