ADP3000AN AD [Analog Devices], ADP3000AN Datasheet - Page 7

• Manufacturer Part Number: ADP3000AN
• Description: Micropower Step-Up/Step-Down Fixed 3.3 V, 5 V, 12 V and Adjustable High Frequency Switching Regulator
• Manufacturer: AD [Analog Devices]

Vendor
AVX
Sanyo
Sprague
Panasonic
DIODE SELECTION
The ADP3000’s high switching speed demands the use of
Schottky diodes. Suitable choices include the 1N5817, 1N5818,
1N5819, MBRS120LT3 and MBR0520LT1. Do not use fast
recovery diodes because their high forward drop lowers effi-
ciency. Neither general-purpose diodes nor small signal diodes
should be used.
PROGRAMMING THE SWITCHING CURRENT LIMIT
OF THE POWER SWITCH
The ADP3000’s R
current limit to be programmed with a single external resistor.
This feature offers major advantages which ultimately decrease
the component cost and P.C.B. real estate. First, it allows the
ADP3000 to use low value, low saturation current and physi-
cally small inductors. Additionally, it allows the ADP3000 to
use a physically small surface mount tantalum capacitor with a
typical ESR of 0.1
mV to 80 mV, as well as low input ripple.
As a rule of thumb, the current limit is usually set to approximately
3 to 5 times the full load current for boost applications and
about 1.5–3 times of the full load current in buck applications.
The internal structure of the I
Q1 is the ADP3000’s internal power switch, which is paralleled
by sense transistor Q2. The relative sizes of Q1 and Q2 are
scaled so that IQ2 is 0.5% of IQ1. Current flows to Q2 through
both an internal 80
on these two resistors biases the base-emitter junction of the
oscillator-disable transistor, Q3. When the voltage across R1
and R
pulse. If only the 80
is connected directly to V
be 1.5 A. Figure 8a gives values for lower current-limit values.
REV. 0
LIM
Figure 17. ADP3000 Current Limit Operation
exceeds 0.6 V, Q3 turns on and terminates the output
V
IN
Table II. Recommended Capacitors
Series
TPS
OS-CON
595D
HFQ
ADP3000
LIM
400kHz
V
to achieve an output ripple as low as 40
resistor and the R
OSC
pin permits the cycle by cycle switch
IN
internal resistor is used (i.e. the I
Q3
IN
(EXTERNAL)
), the maximum switch current will
Surface Mount
Through-Hole
Surface Mount
Through-Hole
Type
DRIVER
R
LIM
LIM
I
LIM
R1
circuit is shown in Figure 17.
Q2
200
80
(INTERNAL)
I
Q1
LIM
SW2
SW1
resistor. The voltage
Phone Numbers
[phone]
[phone]
[phone]
[phone]
POWER
SWITCH
Q1
LIM
pin
–7–
The delay through the current limiting circuit is approximately
0.3 s. If the switch ON time is reduced to less than 1.7 s,
accuracy of the current trip-point is reduced. Attempting to
program a switch ON time of 0.3 s or less will produce
spurious responses in the switch ON time. However, the
ADP3000 will still provide a properly regulated output voltage.
PROGRAMMING THE GAIN BLOCK
The gain block of the ADP3000 can be used as a low battery
detector, error amplifier or linear post regulator. The gain block
consists of an op amp with PNP inputs and an open-collector
NPN output. The inverting input is internally connected to the
ADP3000’s 1.245 V reference, while the noninverting input is
available at the SET pin. The NPN output transistor will sink in
excess of 300 A.
Figure 18 shows the gain block configured as a low battery
monitor. Resistors R1 and R2 should be set to high values to
reduce quiescent current, but not so high that bias current in
the SET input causes large errors. A value of 33 k for R2 is a
good compromise. The value for R1 is then calculated from the
formula:
where V
gain block output is an open-collector NPN, a pull-up resistor
should be connected to the positive logic power supply.
Figure 18. Setting the Low Battery Detector Trip Point
LOBATT
V
BATT
is the desired low battery trip point. Since the
33k
R2
R1
R1
V
SET
LOBATT
ADP3000
1.245V
REF
R1 =
V
1.245 V
LB
= BATTERY TRIP POINT
R2
V
GND
– 1.245 V
LB
37.7µA
– 1.245V
1.6M
V
R
IN
HYS
AO
5V
R
47k
ADP3000
TO
PROCESSOR
L