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ADP1111ARZ
Analog Devices
ADP1111ARZ Datasheet PDF : 15 Pages
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ADP1111
CIRCUIT OPERATION, STEP DOWN (BUCK) MODE)
The ADP1111’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 V
logic supply.
A typical configuration for step down operation of the ADP1111
is shown in Figure 19. 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 towards
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
≅ VIN
− VCE
L
− VOUT
• 7μs
where 7 μs is the ADP1111 switch’s “on” time.
VIN
+
C2
RLIM
100Ω
123
ILIM VIN SW1
FB 8
ADP1111
SW2 4
AO SET GND
675
NC NC
L1
D1 +
1N5818
R2
CL
R1
VOUT
Figure 19. 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 ADP1111’s SW2 pin is 0.5 V below
ground. To avoid exceeding this limit, D1 must be a Schottky
diode. If a silicon diode is used for D1, Pin SW2 can go to
–0.8 V, which will cause potentially damaging power dissipation
within the ADP1111.
The output voltage of the buck regulator is fed back to the
ADP1111’s FB pin by resistors R1 and R2. When the voltage at
pin FB falls below 1.25 V, the internal power switch turns “on”
again, and the cycle repeats. The output voltage is set by the
formula:
VOUT
= 1.25 V
•
⎛
⎝⎜1
+
R2⎞
R1 ⎠⎟
When operating the ADP1111 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 20.
VIN
+
C2
R3
123
ILIM
VIN
SW1
FB 8
ADP1111
D2
L1
R2
SW2 4
GND
+
5
D1 C1
R1
VOUT
D1, D2 = 1N5818 SCHOTTKY DIODES
Figure 20. Step-Down Model, VOUT > 6.2 V
If the input voltage to the ADP1111 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.
INCREASING OUTPUT CURRENT IN THE STEP-DOWN
REGULATOR
Unlike the boost configuration, the ADP1111’s internal power
switch is not saturated when operating in step-down mode. A
conservative value for the voltage across the switch in step-down
mode is 1.5 V. This results in high power dissipation within the
ADP1111 when high peak current is required. To increase the
output current, an external PNP switch can be added (Figure
21). In this circuit, the ADP1111 provides base drive to Q1
through R3, while R4 ensures that Q1 turns off rapidly. Because
the ADP1111’s internal current limiting function will not work
in this circuit, R5 is provided for this purpose. With the value
shown, R5 limits current to 2 A. In addition to reducing power
dissipation on the ADP1111, this circuit also reduces the switch
voltage. When selecting an inductor value for the circuit of
Figure 21, the switch voltage can be calculated from the
formula:
VSW = VR5 + VQ1(SAT) ≅ 0.6 V + 0.4 V ≅ 1 V
INPUT
+
CINPUT
R5
0.3Ω
1
ILIM
R4
220Ω
R3
330Ω
SW1 3
Q1
MJE210
L1
2 VIN ADP1111
R2
FB 8
AO SET GND SW2
6 754
D1
R1
1N5821
NC NC
OUTPUT
+
CL
Figure 21. High Current Step-Down Operation
–10–
RREEVV. A0
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