
Part Name  Description  Manufacturer 
ADP1110AR5  Micropower, StepUp/StepDown Switching Regulator; Adjustable and Fixed 3.3 V, 5 V, 12 V  Analog Devices 
ADP1110AR5 Datasheet PDF : 16 Pages

ADP1110
As previously mentioned, the switch voltage is higher in step
down mode than in stepup mode. VSW is a function of switch
current and is therefore a function of VIN, L, time and VOUT.
For most applications, a VSW value of 1.5 V is recommended.
The inductor value can now be calculated:
L
= V IN(MIN) –V SW
I PEAK
– V OUT
• tON
(Equation 7)
where: tON = Switch ON time (10 µs)
If the input voltage will vary (such as an application that must
operate from a 9 V, 12 V or 15 V source), an RLIM resistor
should be selected from Figure 6. The RLIM resistor will keep
switch current constant as the input voltage rises. Note that
there are separate RLIM values for stepup and stepdown modes
of operation.
For example, assume that +5 V at 250 mA is required from a
+9 V to +18 V source. Deriving the peak current from Equation
6 yields:
I PEAK
=
2
•250 mA
0.69
9
5+ 0.5
−1.5+ 0.5
=
498
mA
Then, the peak current can be inserted into Equation 7 to
calculate the inductor value:
L = 9 –1.5 – 5 •10 µs = 50 µs
498 mA
Since 50 µH is not a standard value, the next lower standard
value of 47 µH would be specified.
To avoid exceeding the maximum switch current when the
input voltage is at +18 V, an RLIM resistor should be specified.
Using the stepdown curve of Figure 6, a value of 560 Ω will
limit the switch current to 500 mA.
INDUCTOR SELECTION—POSITIVETONEGATIVE
CONVERTER
The configuration for a positivetonegative converter using the
ADP1110 is shown in Figure 23. As with the stepup converter,
all of the output power for the inverting circuit must be supplied
by the inductor. The required inductor power is derived from
the formula:
( ) ( ) PL = VOUT + VD • IOUT
(Equation 8)
The ADP1110 power switch does not saturate in positiveto
negative mode. The voltage drop across the switch can be
modeled as a 0.75 V baseemitter diode in series with a 0.65 Ω
resistor. When the switch turns on, inductor current will rise at
a rate determined by:
IL
(t
)
=
VL
R'
1−
–R't
eL
(Equation 9)
where: R' = 0.65 Ω + RL(DC)
VL = VIN – 0.75 V
For example, assume that a –5 V output at 75 mA is to be
generated from a +4.5 V to +5.5 V source. The power in the
inductor is calculated from Equation 8:
PL = (– 5V+ 0.5V ) •(75 mA) = 413 mW
During each switching cycle, the inductor must supply the
following energy:
PL = 413 mW = 5.9 µJ
f OSC 70 kHz
Using a standard inductor value of 56 µH with 0.2 Ω dc
resistance will produce a peak switch current of:
I PEAK
=
4.5V – 0.75V
0.65 Ω + 0.2 Ω
–0.85 Ω •10 µs
1− e 56 µH
= 621 mA
Once the peak current is known, the inductor energy can be
calculated from Equation 9:
EL
=
1
2
(56
µH ) • (621 mA)2
=10.8 µJ
Since the inductor energy of 10.8 µJ is greater than the PL/fOSC
requirement of 5.9 µJ, the 56 µH inductor will work in this
application.
The input voltage only varies between 4.5 V and 5.5 V in this
example. Therefore, the peak current will not change enough to
require an RLIM resistor and the ILIM pin can be connected
directly to VIN. Care should be taken, of course, to ensure that
the peak current does not exceed 800 mA.
CAPACITOR SELECTION
For optimum performance, the ADP1110’s output capacitor
must be selected carefully. Choosing an inappropriate capacitor
can result in low efficiency and/or high output ripple.
Ordinary aluminum electrolytic capacitors are inexpensive but
often have poor Equivalent Series Resistance (ESR) and
Equivalent Series Inductance (ESL). Low ESR aluminum
capacitors, specifically designed for switch mode converter
applications, are also available, and these are a better choice
than general purpose devices. Even better performance can be
achieved with tantalum capacitors, although their cost is higher.
Very low values of ESR can be achieved by using OSCON
capacitors (Sanyo Corporation, San Diego, CA). These devices
are fairly small, available with tapeandreel packaging and have
very low ESR.
The effects of capacitor selection on output ripple are demon
strated in Figures 16, 17 and 18. These figures show the output
of the same ADP1110 converter, that was evaluated with three
different output capacitors. In each case, the peak switch
current is 500 mA, and the capacitor value is 100 µF. Figure 16
shows a Panasonic HFseries 16volt radial cap. When the
switch turns off, the output voltage jumps by about 90 mV and
then decays as the inductor discharges into the capacitor. The
rise in voltage indicates an ESR of about 0.18 Ω. In Figure 17,
the aluminum electrolytic has been replaced by a Sprague 293D
series, a 6 V tantalum device. In this case the output jumps
–8–
REV. 0

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