ADP3820

–7–

REV. A

The current sense resistor for this application:

Ω

FET Selection

The type and size of the pass transistor are determined by the

threshold voltage, input-output voltage differential and load

current. The selected PMOS must satisfy the physical and ther-

provided by the controller will turn on the FET at worst case

conditions, (i.e., temperature and manufacturing tolerances) the

calculated as follows:

× R

S

where

= Current Sense Resistor

~ 0.7 V (Room Temperature)

~ 0.5 V (Hot)

~ 0.9 V (Cold)

For example:

× 50 m

Ω = 4.25 V

used.

voltage drop due to the sense resistor plus the ON-resistance

of the FET at the maximum charge current. The selected

MOSFET must satisfy these criteria; otherwise, a different pass

device should be used.

From the Drain-to Source current vs. Drain-to-Source voltage

vs. gate drive graph off the MOSFET data sheet, it can be de-

indicates. However, the value read from the MOSFET data

sheet graph must be adjusted based on the junction temperature

of the MOSFET. This adjustment factor can be obtained from

MOSFET data sheet.

External Capacitors

The ADP3820 is stable with or without a battery load, and

virtually any good quality output filter capacitors can be used

(anyCAP™), independent of the capacitor’s minimum ESR

(Effective Series Resistance) value. The actual value of the

tance of the external PMOS device. A 10

µF tantalum or alumi-

num electrolytic capacitor at the output is sufficient to ensure

stability for up to a 10 A output current.

Shutdown Mode

Applying a TTL high signal to the

SD pin or tying it to the

input pin will enable the output. Pulling this pin low or tying

it to ground will disable the output. In shutdown mode, the

controller’s quiescent current is reduced to less than 1

µA.

Gate-to-Source Clamp

A 6 V gate-to-source voltage clamp is provided by the ADP3820 to

and the output is suddenly shorted to ground. This allows use of

Short Circuit Protection

The power FET is protected during short circuit conditions

with a foldback type of current limiting that significantly re-

duces the current. See Figure 13 for foldback current limit

information.

Current Sense Resistor

Current limit is achieved by setting an appropriate current sense

derating is advised to select the power dissipation rating of the

resistor.

The simplest and cheapest sense resistor for high current appli-

cations, (i.e., Figure 1) is a PCB trace. However, the tempera-

ture dependence of the copper trace and the thickness tolerances of

the trace must be considered in the design. The resistivity of

copper has a positive temperature coefficient of +0.39%/

°C.

Copper’s Tempco, in conjunction with the proportional-to-

absolute temperature (

±0.3%) current limit voltage, can provide

an accurate current limit. Table I provides the typical resistance

values for PCB copper traces. Alternately, an appropriate sense

resistor, such as surface mount sense resistors, available from

KRL, can be used.

Table I. Printed Circuit Copper Resistance

Conductor

Conductor

Resistance

Thickness

Width/Inch

m /In

(18

µm)

0.025

39.3

0.050

19.7

0.100

9.83

0.200

4.91

0.500

1.97

(35

µm)

0.025

19.7

0.050

9.83

0.100

4.91

0.200

2.46

0.500

0.98

(70

µm)

0.025

9.83

0.050

4.91

0.100

2.46

0.200

1.23

0.500

0.49

(106

µm)

0.025

6.5

0.050

3.25

0.100

1.63

0.200

0.81

0.500

0.325

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