ADP3806 Analog Devices, Inc., ADP3806 Datasheet - Page 15

• Manufacturer Part Number: ADP3806
• Description: High-frequency Switch Mode Li-ion Battery Charger
• Manufacturer: Analog Devices, Inc.

Current-Sense Filtering
During normal circuit operation, the current-sense signals can
have high frequency transients that need filtering to ensure
proper operation. In the case of the CS+ and CS− inputs, the
resistors (R3 and R4) are set to 249 Ω and the filter capacitor
(C13) value is 22 nF. For the system current sense circuits,
common-mode filtering from SYS+ and SYS− to ground is
needed. 470 nF ceramic capacitors (C1, C2) with 2.2 Ω resistors
(R1, R2) usually suffice. These time constants can be adjusted in
the laboratory if required but represent a good starting point.
MOSFET Selection
One of the features of the ADP3806 is that it allows use of a
high-side NMOS switch instead of a more costly PMOS device.
The converter also uses synchronous rectification for optimal
efficiency. In order to use a high-side NMOS, an internal
bootstrap regulator automatically generates a 7 V supply
across C9.
Maximum output current determines the R
for the two power MOSFETs. When the ADP3806 is operating
in continuous mode, the simplifying assumption can be made
that one of the two MOSFETs is always conducting the load
current. The power dissipation for each MOSFET is given by:
DS(ON)
requirement
Rev. C | Page 15 of 16
Upper MOS
P
Lower MOS
P
where f is the switching frequency and T
transition time, usually 10 ns. The first term accounts for
conduction losses and the second term estimates switching
losses. Using these equations and the manufacturer’s data
sheets, the proper device can be selected.
A Schottky diode (D1) in parallel with Q2 conducts only during
dead time between the two power MOSFETs. The purpose of
the D1 is to prevent the body diode of the lower N-channel
MOSFET from turning on, which could cost as much as 1% in
efficiency. One option is to use a combined MOSFET with the
Schottky diode in a single package; these integrated packages
often work better in practice. Examples are the IRF7807D2 and
the Si4832.
DISS
DISS
= R
= R
DS(ON)
DS(ON)
× (I
× (I
BAT
BAT
× √D)
× √D)
2
2
+ V
+ V
IN
IN
× I
× (I
BAT
BAT
× √D × T
SW
× √I − D)
is the switch
SW
ADP3806
2
× T
× f
SW
× f(12)
(11)