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ADP3000-1997 View Datasheet(PDF) - Analog Devices

Part NameADP3000(1997) ADI
Analog Devices ADI
DescriptionMicropower Step-Up/Step-Down Fixed 3.3 V, 5 V, 12 V, Adjustable High Frequency Switching Regulator
ADP3000 Datasheet PDF : 12 Pages
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ADP3000
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 RHYS,
with a value of 1 Mto 10 M, provides the hysteresis. The
addition of RHYS will change the trip point slightly, so the new
value for R1 will be:
R1= V LOBATT –1.245V
1.245
 R2
V

VRLL−+1.R2H45YSV

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).
VIN
+
C2
D1, D2 = 1N5818 SCHOTTKY DIODES
R3
1 23
ILIM VIN SW1
FB 8
ADP3000
D2
GND
SW2 4
5
D1
VOUT > 6V
L1
R2
+
C1
R1
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
2R
+V
IN ISW
β
D
1–VVION

4IO
ISW

+
IQ
V IN
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 (Typical RCE(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
+
1
β

VIN
VO
VCE(SAT )


2 IO
ISW

+
IQ
VIN
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:
where:
T = PD × θ JA
T = Temperature Rise.
PD = Device Power Dissipation.
θJA = 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)
30

0.75
1 –
2
3.3 
(4) 0.18
 0.8

+
500 E 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.
–8–
REV. 0
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