ADP1073AN-5 AD [Analog Devices], ADP1073AN-5 Datasheet - Page 9

• Manufacturer Part Number: ADP1073AN-5
• Description: Micropower DC.DC Converter Adjustable and Fixed 3.3 V, 5 V, 12 V
• Manufacturer: AD [Analog Devices]

Circuit Operation, Step-Up (Boost) Mode
In boost mode, the ADP1073 produces an output voltage that is
higher than the input voltage. For example, +5 V can be
derived from one alkaline cell (+1.5 V), or +12 V can be
generated from a +5 V logic power supply.
Figure 15 shows an ADP1073 configured for step-up operation.
The collector of the internal power switch is connected to the
output side of the inductor, while the emitter is connected to
GND. When the switch turns on, Pin SW1 is pulled near ground.
This action forces a voltage across L1 equal to V
current begins to flow through L1. This current reaches a final
value (ignoring second-order effects) of:
where 38 s is the ADP1073 switch’s “on” time.
When the switch turns off, the magnetic field collapses. The
polarity across the inductor changes, current begins to flow
through D1 into the load and the output voltage is driven above
the input voltage.
The output voltage is fed back to the ADP1073 via resistors R1
and R2. When the voltage at pin FB falls below 212 mV, SW1
turns “on” again and the cycle repeats. The output voltage is
therefore set by the formula:
The circuit of Figure 15 shows a direct current path from V
V
is not protected if the output is short circuited to ground.
Circuit Operation, Step-Down (Buck) Mode)
The ADP1073’s step-down mode is used to produce an output
voltage that is lower than the input voltage. For example, the
output of four NiCd cells (+4.8 V) can be converted to a +3.3 V
logic supply.
A typical configuration for step-down operation of the ADP1073
is shown in Figure 16. In this case, the collector of the internal
power switch is connected to V
inductor. When the switch turns on, SW2 is pulled up toward
V
and causes current to flow in L1. This current reaches a final
value of:
where 38 s is the ADP1073 switch’s “on” time.
REV. 0
OUT
IN
. This forces a voltage across L1 equal to (V
, via the inductor and D1. Therefore, the boost converter
V
IN
*OPTIONAL
Figure 15. Step-Up Mode Operation
V
I
I
PEAK
PEAK
OUT
GND
I
LIM
1
5
ADP1073
R3*
212 mV
V
V
SW2
V
2
4
IN
IN
IN
–V
–V
SW1
FB
L
CE
L
CE(SAT )
L1
3
8
IN
–V
1
and the emitter drives the
R2
OUT
R1
D1
38 s
38 s
R1
R2
IN
IN
C1
– V
– V
V
CE
OUT
CE(SAT)
) – V
OUT
and
IN
,
to
–9–
When the switch turns off, the magnetic field collapses. The
polarity across the inductor changes and the switch side of the
inductor is driven below ground. Schottky diode D1 then turns
on and current flows into the load. Notice that the Absolute
Maximum Rating for the ADP1073’s SW2 pin is 0.5 V below
ground. To avoid exceeding this limit, D1 must be a Schottky
diode. Using a silicon diode in this application will generate
forward voltages above 0.5 V, which will cause potentially dam-
aging power dissipation within the ADP1073.
The output voltage of the buck regulator is fed back to the
ADP1073’s FB pin by resistors R1 and R2. When the voltage at
pin FB falls below 212 mV, the internal power switch turns
“on” again and the cycle repeats. The output voltage is set by
the formula:
The output voltage should be limited to 6.2 V or less when
using the ADP1073 in step-down mode.
If the input voltage to the ADP1073 varies over a wide range, a
current limiting resistor at Pin 1 may be required. If a particular
circuit requires high peak inductor current with minimum input
supply voltage the peak current may exceed the switch maximum
rating and/or saturate the inductor when the supply voltage is at the
maximum value. See the Limiting the Switch Current section of
this data sheet for specific recommendations.
Positive-to-Negative Conversion
The ADP1073 can convert a positive input voltage to a negative
output voltage, as shown in Figure 17. This circuit is essentially
identical to the step-down application of Figure 16, except that
the “output” side of the inductor is connected to power ground.
When the ADP1073’s internal power switch turns off, current
flowing in the inductor forces the output (–V
potential. The ADP1073 will continue to turn the switch on
until its FB pin is 212 mV above its GND pin, so the output
voltage is determined by the formula:
V
V
IN
IN
C2
C2
Figure 17. A Positive-to-Negative Converter
Figure 16. Step-Down Mode Operation
I
I
LIM
LIM
1
R3
R3
220
ADP1073
ADP1073
GND
V
GND
IN
V
V
2
5
IN
OUT
SW1
SW1
3
SW2
SW2
FB
212 mV
FB
8
4
D1
1N5818
D1
1N5818
L1
L1
1
R2
R1
OUT
C1
ADP1073
C1
) to a negative
R2
R1
R1
R2
V
OUT
V
OUT