ADP1111
As previously mentioned, the switch voltage is higher in step-
down mode than in step-up mode. VSW is a function of switch
During each switching cycle, the inductor must supply the
following energy:
current and is therefore a function of VIN, L, time and VOUT
.
PL 275mW
For most applications, a VSW value of 1.5 V is recommended.
=
= 3.8 μJ
fOSC 72kHz
The inductor value can now be calculated:
Using a standard inductor value of 56 μH with 0.2 Ω dc
resistance will produce a peak switch current of:
VIN MIN −VSW −VOUT
(
)
L =
•tON
(Equation 7)
IPEAK
−0.85 Ω•7μs
56 μH
⎛
⎞
4.5V − 0.75V
0.65 Ω+0.2 Ω
where tON = switch ON time (7 μs).
IPEAK
=
1−e
= 445mA
⎜
⎟
⎝
⎠
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 step-up and step-down modes
of operation.
Once the peak current is known, the inductor energy can be
calculated from (Equation 9):
1
EL
=
56 μH • 445mA 2 = 5.54 μJ
(
) (
)
2
For example, assume that +5 V at 300 mA is required from a
+12 V to +24 V source. Deriving the peak current from
Equation 6 yields:
Since the inductor energy of 5.54 μJ is greater than the PL/fOSC
requirement of 3.82 μJ, the 56 μH inductor will work in this
application.
2•300 mA⎛ 5+ 0.5
⎞
⎟
IPEAK
=
= 600 mA
The input voltage only varies between 4.5 V and 5.5 V in this
application. 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 650 mA.
⎜
0.5
12 −1.5 + 0.5
⎝
⎠
Then, the peak current can be inserted into Equation 7 to
calculate the inductor value:
12−1.5 − 5
600 mA
L =
• 7μs = 64 μH
CAPACITOR SELECTION
For optimum performance, the ADP1111’s output capacitor
must be selected carefully. Choosing an inappropriate capacitor
can result in low efficiency and/or high output ripple.
Since 64 μH is not a standard value, the next lower standard
value of 56 μH would be specified.
To avoid exceeding the maximum switch current when the
input voltage is at +24 V, an RLIM resistor should be specified.
Using the step-down curve of Figure 6, a value of 560 Ω will
limit the switch current to 600 mA.
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 OS-CON
capacitors (Sanyo Corporation, San Diego, CA). These devices
are fairly small, available with tape-and-reel packaging and have
very low ESR.
INDUCTOR SELECTION–POSITIVE-TO-NEGATIVE
CONVERTER
The configuration for a positive-to-negative converter using the
ADP1111 is shown in Figure 22. As with the step-up 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:
The effects of capacitor selection on output ripple are demon-
strated in Figures 15, 16 and 17. These figures show the output
of the same ADP1111 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 15
shows a Panasonic HF-series 16-volt 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 16,
the aluminum electrolytic has been replaced by a Sprague 293D
series, a 6 V tantalum device. In this case the output jumps
about 30 mV, which indicates an ESR of 0.06 Ω. Figure 17
shows an OS-CON 16–volt capacitor in the same circuit, and
ESR is only 0.02 Ω.
PL
=
V
+ VD • I
(Equation 8)
(
)
(
)
OUT
OUT
The ADP1111 power switch does not saturate in positive-to-
negative mode. The voltage drop across the switch can be
modeled as a 0.75 V base-emitter diode in series with a 0.65 Ω
resistor. When the switch turns on, inductor current will rise at
a rate determined by:
−R't
L
⎛
⎞
VL
R'
IL t =
1− e
( )
(Equation 9)
⎜
⎟
⎝
⎠
where: R' = 0.65 Ω + RL(DC)
VL = VIN – 0.75 V
For example, assume that a –5 V output at 50 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| • 50mA = 275mW
(
) (
)
REV.
A
REV. 0
–8–
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