ADP1073 データシートの表示（PDF） - Analog Devices
部品番号
コンポーネント説明
メーカー
ADP1073 Datasheet PDF : 16 Pages
| |||
ADP1073
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 VIN – VCE(SAT) and
current begins to flow through L1. This current reaches a final
value (ignoring second-order effects) of:
I PEAK
≅V IN
–V CE(SAT ) × 38 µs
L
where 38 µs is the ADP1073 switch’s “on” time.
L1
VIN
R3*
1
2
ILIM
VIN
ADP1073
GND
5
SW2
4
SW1 3
FB 8
D1
VOUT
R1
C1
R2
*OPTIONAL
Figure 15. Step-Up Mode Operation
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:
V OUT
= 212 mV
×
1+
R1
R2
The circuit of Figure 15 shows a direct current path from VIN to
VOUT, via the inductor and D1. Therefore, the boost converter
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 VIN and the emitter drives the
inductor. When the switch turns on, SW2 is pulled up toward
VIN. This forces a voltage across L1 equal to (VIN – VCE) – VOUT,
and causes current to flow in L1. This current reaches a final
value of:
I PEAK
≅V IN
–V CE
L
–V OUT
× 38 µs
where 38 µs is the ADP1073 switch’s “on” time.
VIN
C2
R3
220⍀
1
2
3
ILIM
VIN SW1 FB 8
ADP1073
SW2 4
GND
5
L1
D1
1N5818
C1
VOUT
R1
R2
Figure 16. Step-Down Mode Operation
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:
V OUT
= 212 mV
×
1+
R1
R2
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 (–VOUT) to a negative
؉VIN
R3
ILIM VIN SW1 FB
C2
ADP1073
SW2
GND
L1
D1
1N5818
C1
R2
R1
؊VOUT
Figure 17. A Positive-to-Negative Converter
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:
REV. 0
–9–
Share Link: 