ADP3801 datasheet(14/20 Pages) AD

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ADP3801/ADP3802

–14–

REV. 0

Gate Drive

The ADP3801 and ADP3802 are designed to directly drive the

gate of a PFET with no additional circuitry as shown on the

circuit diagram. The DRV pin pulls the gate up to within

250 mV of VCC, which is more than enough to ensure that the

transistor turns off. To turn the PFET on, the DRV pin pulls

down to a clamped voltage that is at most 7 V below VCC. Check

the specified PFET’s maximum Gate-Source rating to see if this

voltage does not exceed its breakdown. The Si4463 is rated at

VGSMAX = 12 V, which is well above the maximum gate drive for

the ADP3801/ADP3802.

Schottky Rectifier Selection and Thermal Design

The Schottky diode’s peak current and average power dissipa-

tion must not exceed the diode ratings. The most stressful con-

dition for the output diode is under short circuit (VO = 0 V),

where the diode duty cycle DD is at least 95%. Under this con-

dition, the diode must safely handle IPK at close to 100% duty

cycle.

The diode power dissipation (PD) is calculated by multiplying

the forward voltage drop (VF) times the Schottky diode duty

cycle multiplied by the short circuit current. The worst-case

forward voltage drop of MBRD835 diode is 0.41 V at IPK =

4.5 A, thus:

PD = IPK × DD × VF = 4.5 × 0.95 × 0.41 = 1.8W

From the diode’s worst-case dissipation, the maximum junction

temperature TJMAX of the diode can be calculated:

TJMAX = TA + RθJA × PD = 25 + (40) × 1.8 = 97°C

RθJA is the junction to ambient thermal impedance of the diode.

The calculated TJMAX should be below the maximum allowed

junction temperature of the diode with adequate margin. TJMAX

of the MBRD835 is 125

°C, which is met with adequate margin.

Input Capacitor Selection

In continuous mode, the source current of the PMOS is a square

wave of duty cycle VOUT/VIN. To prevent large voltage tran-

sients, a low ESR input capacitor sized for the maximum rms

current must be used. The maximum rms capacitor current is

given by:

RMS

OUT

≈×

−

()

This formula has a maximum at

VIN = 2 VOUT, where IRMS =

IOUT/2 = 2.0 A.

This simple worst case condition is commonly used for design

because even significant deviations do not offer much relief.

Note that capacitor manufacturers’ ripple current ratings are

often based on only 2000 hours of life. This makes it advisable

to further derate the capacitor, or to choose a capacitor rated at

a higher temperature than required. Several capacitors may also

be paralleled to meet size or height requirements in the design.

As a first choice, three 68

µF/20 V Sprague type 593D tantalum

capacitors are used in parallel. Each is specified as follows:

ESR = 0.2

Ω, maximum ripple current of 0.91A

RMS. In addition

to CIN, a 0.1 µF decoupling capacitor is required as close as

possible to the VCC pin.

Once the capacitor is chosen, the input ripple voltage should be

checked:

ESR

kHz

I NRIPPLE

OSC

=×













=×













100

007

84 100

200

210

360

Ω

This is low enough for most applications. For cost reduction,

one of the 68

µF capacitors could be removed, or a cheaper

electrolytic could be used instead.

Output Capacitor Selection

As a first choice, we’ll use two of the same type of 68

µF

Sprague capacitors for the output. The inductor rms ripple current

was calculated as 0.3 A, which is far below the specification for

these capacitors. The other consideration is the allowable output

ripple voltage. Assuming high battery internal resistance, all of

the worst case inductor ripple current may flow through the

output capacitor. This results in a ripple voltage of:

ESR

kHz

OUTRIPPLE

LPP

OSC

OUT

=×













=×













100

84 100

200

140

130

Ω

This ripple is low enough for most applications. Again, one of

the capacitors could be removed or lower cost electrolytic ca-

pacitors could be used to reduce cost.

Charger Performance Summary

The circuit properly executes the charging algorithm, exhibiting

stable operation regardless of battery conditions, including an

open circuit load in which the battery is removed.

Li-Ion charging characteristics are given in Figure 25. The

charge current is maintained at its programmed level until the

battery reaches its final voltage. Then the current begins to

decrease. The shape of the current decrease is dependent on the

internal impedance of the battery. When the current drops below

240 mA, the EOC comparator signals the end-of-charge of the

battery.

TIME – Hours

0.5

1.0

1.5

2.0

3.0

3.5

4.0

2.5

1.3

2.7

4.0

2.5

4.5

VBAT

ICHARGE

EOC

Figure 25. Li-Ion Charging Characteristic

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