ADP1173 Просмотр технического описания (PDF) - Analog Devices
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ADP1173 Datasheet PDF : 16 Pages
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ADP1173
L1
VIN
R3*
1
2
ILIM VIN SW1 3
ADP1173
GND SW2 FB 8
5
4
D1
VOUT
R1
+
C1
R2
* = OPTIONAL
Figure 14. 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 ADP1173 via resistors R1
and R2. When the voltage at pin FB falls below 1.245 V, SW1
turns “on” again and the cycle repeats. The output voltage is
therefore set by the formula:
V OUT
= 1.245
V
×
1+
R1
R2
The circuit of Figure 14 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 ADP1173’s step-down mode is used to produce an output
voltage 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 ADP1173
is shown in Figure 15. 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
× 23 µs
where 23 µs is the ADP1173 switch’s “on” time.
VIN
R3
100Ω
+
C2
1
2
3
ILIM VIN SW1
FB 8
ADP1173
GND SW2 4
5
L1
+
C1
D1
1N5818
VOUT
R1
R2
Figure 15. 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 ADP1173’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
damaging power dissipation within the ADP1173.
The output voltage of the buck regulator is fed back to the
ADP1173’s FB pin by resistors R1 and R2. When the voltage at
pin FB falls below 1.245 V, the internal power switch turns
“on” again and the cycle repeats. The output voltage is set by
the formula:
V OUT
= 1.245
V
×
1+
R1
R2
When operating the ADP1173 in step-down mode, the output
voltage is impressed across the internal power switch’s emitter-
base junction when the switch is off. To protect the switch, the
output voltage should be limited to 6.2 V or less. If a higher
output voltage is required, a Schottky diode should be placed in
series with SW2, as shown in Figure 16.
If high output current is required in a step-down converter, the
ADP1111 or ADP3000 should be considered. These devices
offer higher frequency operation, which reduces inductor size,
and an external pass transistor can be added to reduce RON of
the switch.
VIN
RLIM
100Ω
+
C2
1
2
3
ILIM VIN SW1
FB 8
ADP1173
1N5818 L1
GND
5
SW2 4
+
C1
D1
1N5818
VOUT
R1
R2
Figure 16. Step-Down Mode, VOUT > 6.2 V
If the input voltage to the ADP1173 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 maxi-
mum 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 ADP1173 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 15, except that
the “output” side of the inductor is connected to power ground.
When the ADP1173’s internal power switch turns off, current
flowing in the inductor forces the output (–VOUT) to a negative
potential. The ADP1173 will continue to turn the switch on
REV. 0
–9–
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