Applications Information
MOSFET Configuration
The MAX4843–MAX4846 can be used with either a sin-
gle MOSFET configuration as shown in the
Typical
Operating Circuit
, or can be configured with a back-to-
back MOSFET as shown in Figure 5. The back-to-back
configuration has almost zero reverse current when the
input supply is below the output.
If reverse current leakage is not a concern, a single
MOSFET can be used. This approach has half the loss of
the back-to-back configuration when used with similar
MOSFET types and is a lower cost solution. Note that if
the input is actually pulled low, the output is also pulled
low due to the parasitic body diode in the MOSFET. If
this is a concern, the back-to-back configuration should
be used.
In a typical application of the MAX4846, an external
adapter with built-in battery charger is connected to IN
and a battery is connected to the source of the external
FET. When the adapter is unplugged, IN is directly con-
nected to the battery through the external FET. Since
the battery voltage is typically greater than 3V, the
GATE voltage stays high and the device remains pow-
ered by the battery.
MOSFET Selection
The MAX4843–MAX4846 are designed for use with
either a single n-channel MOSFET or dual back-to-back
n-channel MOSFETs. In most situations, MOSFETs with
R
ON
specified for a V
GS
of 4.5V work well. If the input
supply is near the UVLO maximum of 3.5V, consider
using a MOSFET specified for a lower V
GS
voltage.
Also the V
DS
should be 30V for the MOSFET to with-
stand the full 28V IN range of the MAX4843–MAX4846.
Table 1 shows a selection of MOSFETs appropriate for
use with the MAX4843–MAX4846.
IN Bypass Considerations
For most applications, bypass IN to GND with a 1µF
ceramic capacitor. If the power source has significant
inductance due to long lead length, take care to pre-
vent overshoots due to the LC tank circuit and provide
protection if necessary to prevent exceeding the 30V
absolute maximum rating on IN.
The MAX4843–MAX4846 provide protection against
voltage faults up to 28V, but this does not include nega-
tive voltages. If negative voltages are a concern, con-
nect a Schottky diode from IN to GND to clamp
negative input voltages.
ESD Test Conditions
ESD performance depends on a number of conditions.
The MAX4843–MAX4846 are protected from ±15kV typ-
ical ESD on IN when IN is bypassed to ground with a
1µF ceramic capacitor.
MAX4843–MAX4846
Overvoltage Protection Controllers with
Low Standby Current
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