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ADP1108AR-5 View Datasheet(PDF) - Analog Devices

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ADP1108AR-5 Datasheet PDF : 12 Pages
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ADP1108
where: DC = duty cycle (0.7 for the ADP1108)
VSW = voltage drop across the switch
VD = diode drop (0.5 V for a 1N5818)
IOUT = output current
VOUT = the output voltage
VIN = the minimum input voltage
As previously mentioned, the switch voltage is higher in step-
down mode than in step-up 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 (36 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 4. 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.
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
0.7

9
5 + 0.5
1.5 + 0.5

= 491 mA
The peak current can than be inserted into Equation 7 to cal-
culate the inductor value:
L
=
9 – 1.5 – 5
491 mA
×
36
µs
=
183
µH
Since 183 µH is not a standard value, the next lower standard
value of 150 µH would be specified.
To avoid exceeding the maximum switch current when the in-
put voltage is at +18 V, an RLIM resistor should be specified. Us-
ing Figure 4, a value of 160 will limit the switch current to
500 mA.
Inductor Selection—Positive-to-Negative Converter
The configuration for a positive-to-negative converter using the
ADP1108 is shown in Figure 19. 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:
( ) ( ) PL |VOUT|+ V D × IOUT
(Equation 8)
The ADP1108 power switch does not saturate in positive-to-
negative mode. The voltage drop across the switch can be mod-
eled as a 0.75 V base-emitter diode in series with a 0.65
resistor. When the switch turns on, inductor current will rise at
a rate determined by:
IL
(t) = VL
R'
R't
1e L 
(Equation 9)
where: R' = 0.65 + RL (DC)
VL = VIN – 0.75 V
For example, assume that a –5 V output at 100 mA is to be gen-
erated from a +4.5 V to +5.5 V source. The power in the induc-
tor is calculated from Equation 8:
PL = (|– 5V|+ 0.5V ) × (100 mA) = 550 mW
During each switching cycle, the inductor must supply the fol-
lowing energy:
PL = 550 mW = 28.9 µ J
f OSC 19 kHz
Using a standard inductor value of 220 µH with 0.3 dc resis-
tance will produce a peak switch current of:
I PEAK
=
4.5V – 0.75 V
0.65 Ω + 0.3
–0.95 Ω × 36 µs
1 e 220 µH  = 568 mA
Once the peak current is known, the inductor energy can be cal-
culated from Equation 9:
( ) EL
=
1
2
 220
µH
×
568 mA
2
=
35.5
µ
J
The inductor energy of 35.5 µJ is greater than the PL/fOSC re-
quirement of 28.9 µJ, so the 220 µH inductor will work in
this application.
To avoid exceeding the maximum switch current when the in-
put voltage is at +5.5 V, an RLIM resistor should be specified.
Referring to Figure 4, a value of 150 is appropriate in this
application.
Capacitor Selection
For optimum performance, the ADP1108’s output capacitor
must be carefully selected. 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 Equiva-
lent 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 Corpora-
tion, San Diego, CA). These devices are fairly small, available
with tape-and-reel packaging, and have very low ESR.
The effects of capacitor selection on output ripple are demon-
strated in Figures 12, 13, and 14. These figures show the output
of the same ADP1108 converter, which 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 12
shows a Panasonic HF-series* radial aluminum electrolytic.
When the switch turns off, the output voltage jumps by about
90 mV and then decays as the inductor discharges into the ca-
pacitor. The rise in voltage indicates an ESR of about 0.18 . In
Figure 13, the aluminum electrolytic has been replaced by a
Sprague 593D-series* tantalum device. In this case the output
jumps about 35 mV, which indicates an ESR of 0.07 . Figure
14 shows an OS-CON SA series capacitor in the same circuit,
and ESR is only 0.02 .
*All trademarks are the property of their respective holders.
REV. 0
–7–
 

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