ADP3000 datasheet(8/12 Pages) AD

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ADP3000

–8–

REV. 0

The circuit of Figure 18 may produce multiple pulses when

approaching the trip point due to noise coupled into the SET

input. To prevent multiple interrupts to the digital logic,

hysteresis can be added to the circuit (Figure 18). Resistor R

HYS,

with a value of 1 M

Ω to 10 MΩ, provides the hysteresis. The

addition of R

HYS will change the trip point slightly, so the new

value for R1 will be:

V LOBATT –1.245V

1.245V









−

V L −1.245V

RL + RHYS









where VL is the logic power supply voltage, RL is the pull-up

resistor, and RHYS creates the hysteresis.

POWER TRANSISTOR PROTECTION DIODE IN STEP-

DOWN CONFIGURATION

When operating the ADP3000 in the step-down mode, the

output voltage is impressed across the internal power switch’s

emitter-base junction when the switch is off. In order to protect

the switch, a Schottky diode must be placed in a series with

SW2 when the output voltage is set to higher than 6 V. Figure

19 shows the proper way to place the protection diode, D2.

The selection of this diode is identical to the step-down commut-

ing diode (see Diode Selection section for information).

ILIM VIN SW1

SW2

GND

ADP3000

VIN

VOUT > 6V

D1, D2 = 1N5818 SCHOTTKY DIODES

Figure 19. Step-Down Model VOUT > 6.0 V

THERMAL CONSIDERATIONS

Power dissipation internal to the ADP3000 can be approximated

with the following equations.

Step-Up

PD = ISW

V IN ISW











D 1–

V IN













4IO

ISW











 + IQ

[] V

[]

where: ISW is ILIMIT in the case of current limit programmed

externally, or maximum inductor current in the case of

current limit not programmed externally.

R = 1

Ω (Typical R

CE(SAT)).

D = 0.75 (Typical Duty Ratio for a Single Switching

Cycle).

VO = Output Voltage.

IO = Output Current.

VIN = Input Voltage.

IQ = 500

µA (Typical Shutdown Quiescent Current).

β = 30 (Typical Forced Beta)

Step-Down

PD = ISW VCESAT 1+





















VIN – VCE SAT

()













2 IO

ISW











 + IQ

[] V

[]

where: ISW is ILIMIT in the case of current limit is programmed

externally or maximum inductor current in the case of

current limit is not programmed eternally.

VCE(SAT) = Check this value by applying ISW to Figure 8b.

1.2 V is typical value.

D = 0.75 (Typical Duty Ratio for a Single Switching

Cycle).

VO = Output Voltage.

IO = Output Current.

VIN = Input Voltage.

IQ = 500 µA (Typical Shutdown Quiescent Current).

β = 30 (Typical Forced Beta).

The temperature rise can be calculated from:

∆T = P

D ×θ JA

where:

∆T = Temperature Rise.

PD = Device Power Dissipation.

JA = Thermal Resistance (Junction-to-Ambient).

As example, consider a boost converter with the following

specifications:

VIN = 2 V, IO = 180 mA, VO = 3.3 V.

ISW = 0.8 A (Externally Programmed).

With Step-Up Power Dissipation Equation:

PD = 0.82 × 1+

(2)(0.8)











 0.75

[] 1– 2

3.3













(4) 0.18

0.8











 + 500E − 6

[] 2[]

= 185 mW

Using the SO-8 Package:

∆T = 185 mW (170°C/W) = 31.5°C.

Using the N-8 Package:

∆T = 185 mW (120°C/W) = 22.2°C.

At a 70

°C ambient, die temperature would be 101.45°C for

SO-8 package and 92.2

°C for N-8 package. These junction

temperatures are well below the maximum recommended

junction temperature of 125

°C.

Finally, the die temperature can be decreased up to 20% by

using a large metal ground plate as ground pickup for the

ADP3000.

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