ADP1108 Analog Devices, ADP1108 Datasheet - Page 9

• Manufacturer Part Number: ADP1108
• Description: Micropower DC-DC Converter Adjustable and Fixed 3.3 V/ 5 V/ 12 V
• Manufacturer: Analog Devices

When the switch turns on, SW2 is pulled up toward V
forces a voltage across L1 equal to (V
current to flow in L1. This current reaches a final value of:
where 36 s is the ADP1108 switch’s “on” time.
When the switch turns off, the magnetic field collapses. The po-
larity across the inductor changes and the switch side of the in-
ductor is driven below ground. Schottky diode D1 then turns on
and current flows into the load. Notice that the Absolute Maxi-
mum Rating for the ADP1108’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
ADP1108.
The output voltage of the buck regulator is fed back to the
ADP1108’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:
When operating the ADP1108 in step-down mode, the output
voltage is impressed across the internal power switch’s emitter-
base junction while the switch is off. To protect the switch, the
output voltage should be limited to 6.2 V or less. If a higher out-
put voltage is required, a Schottky diode should be placed in se-
ries with SW2, as shown in Figure 17.
If the input voltage to the ADP1108 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, then 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.
REV. 0
V
IN
V
C2
IN
C2
Figure 17. Step-Down Model, V
Figure 16. Step-Down Mode Operation
I
LIM
1
I
LIM
R
100
1
ADP1108
I
LIM
R
100
PEAK
ADP1108
LIM
GND
V
V
5
2
IN
GND
V
OUT
2
5
IN
SW1
3
V
SW1
3
SW2
IN
FB
1.245 V
SW2
–V
FB
4
8
CE
8
4
L
D2
–V
IN
D1
1N5818
1
OUT
D1
1N5818
L1
V
CE
L1
R2
R1
)
36 s
OUT
C1
V
> 6.2 V
OUT
C1
IN
, and causes
. This
R1
R2
R1
R2
V
OUT
V
OUT
–9–
Increasing Output Current in the Step-Down Regulator
Unlike the boost configuration, the ADP1108’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
ADP1108 when high peak current is required. To increase the
output current, an external PNP switch can be added (Figure
18). In this circuit, the ADP1108 provides base drive to Q1
through R3 while R4 ensures that Q1 turns off rapidly. The
ADP1108’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 ADP1108, this circuit also reduces the switch
voltage. When selecting an inductor value for the circuit of Fig-
ure 18, the switch voltage can be calculated from the formula:
Positive-to-Negative Conversion
The ADP1108 can convert a positive input voltage to a negative
output voltage, as shown in Figure 19. 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 ADP1108’s internal power switch turns off, current
flowing in the inductor forces the output ( V
potential. The ADP1108 will continue to turn the switch on un-
til its FB pin is 1.245 V above its GND pin, so the output volt-
age is determined by the formula:
The design criteria for the step-down application also apply to
the positive-to-negative converter. The output voltage should be
limited to |6.2 V|, unless a diode is inserted in series with the
SW2 pin (see Figure 17). Also, D1 must again be a Schottky di-
ode to prevent excessive power dissipation in the ADP1108.
V
6.5V TO 20V
IN
C2
Figure 18. High Current Step-Down Operation
Figure 19. A Positive-to-Negative Converter
V
IN
V
C2
I
SW
LIM
1
R3
ADP1108
GND
V
V
V
2
IN
V
2
5
IN
R5
0.22
100
OUT
R5
GND
ADP1108-5
R4
5
SW1
I
V
LIM
1
3
Q1(SAT )
SW2
100
1.245 V
R2
FB
SW2
4
ZTX749
ZETEX
SENSE
SW1
4
Q1
8
3
R3
220
0.6 V
8
D1
1N5818
1
L1
D1
1N5818
*L1 = COILTRONICS CTX100-4
100 H
L1*
R2
R1
0.4 V
ADP1108
C
OUT
L
) to a negative
C1
1V
5V
200mA AT 6.5V
500mA AT 8V
R1
R2
OUT
–V
OUT