ADP3806JRUZ View Datasheet(PDF) - Analog Devices

Part Name

Description

View to exact match

ADP3806JRUZ

High Frequency Switch Mode Li-Ion Battery Charger

Analog Devices 

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 RDS(ON) requirement

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:

ADP3806

Upper MOS

PDISS = RDS(ON) × (IBAT × √D)2 + VIN × IBAT × √D × TSW × f

(11)

Lower MOS

PDISS = RDS(ON) × (IBAT × √D)2 + VIN × (IBAT × √I − D)2 × TSW × f(12)

where f is the switching frequency and TSW is the switch

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.

Rev. C | Page 15 of 16

Share Link: