ADP1173 Analog Devices, ADP1173 Datasheet - Page 6

• Manufacturer Part Number: ADP1173
• Description: Micropower DC-DC Converter
• Manufacturer: Analog Devices

ADP1173
When the internal power switch turns ON, current flow in the
inductor increases at the rate of:
where L is in henrys and R' is the sum of the switch equivalent
resistance (typically 0.8
the inductor. In most applications, where the voltage drop across
the switch is small compared to V
used:
Replacing “t” in the above equation with the ON time of the
ADP1173 (23 s, typical) will define the peak current for a
given inductor value and input voltage. At this point, the
inductor energy can be calculated as follows:
As previously mentioned, E
ADP1173 can deliver the necessary power to the load. For best
efficiency, peak current should be limited to 1 A or less. Higher
switch currents will reduce efficiency, because of increased
saturation voltage in the switch. High peak current also increases
output ripple. As a general rule, keep peak current as low as pos-
sible to minimize losses in the switch, inductor and diode.
In practice, the inductor value is easily selected using the equa-
tions above. For example, consider a supply that will generate
9 V at 50 mA from a 3 V source. The inductor power required
is, from Equation 1:
On each switching cycle, the inductor must supply:
The required inductor power is fairly low in this example, so the
peak current can also be low. Assuming a peak current of 500 mA
as a starting point, Equation 4 can be rearranged to recommend
an inductor value:
Substituting a standard inductor value of 100 H, with 0.2
resistance, will produce a peak switch current of:
Once the peak current is known, the inductor energy can be
calculated from Equation 5:
The inductor energy of 19 J is greater than the P
quirement of 13.5 J, so the 100 H inductor will work in this
application. By substituting other inductor values into the same
equations, the optimum inductor value can be selected.
I
I
E
L
L
L
(t)
(t)
1
2
LI
P
V
V
R
L
L
IN
IN
L
I
2 PEAK
PEAK
E
t
(9V 0.5V – 3V ) (50 mA) 325 mW
L
1– e
I
L(MAX )
V
1
2
f
–R t
P
IN
OSC
(100 H ) (616 mA)
1.0
L
3V
L
t
at +25 C) and the dc resistance of
325 mW
L
24 kHz
1– e
500 mA
must be greater than P
3V
–1.0
IN
100 H
, a simpler equation can be
23 s 138 H
23 s
13.5 J
2
616 mA
19 J
L
L
/f
/f
OSC
OSC
re-
so the
(3)
(4)
(5)
dc
–6–
When selecting an inductor, the peak current must not exceed
the maximum switch current of 1.5 A. If the equations shown
above result in peak currents > 1.5 A, the ADP1073 should be
considered. This device has a 72% duty cycle, so more energy is
stored in the inductor on each cycle. This results in greater
output power.
The peak current must be evaluated for both minimum and
maximum values of input voltage. If the switch current is high
when V
ceeded at the maximum value of V
current limit feature can be used to limit switch current. Simply
select a resistor (using Figure 4) that will limit the maximum
switch current to the I
value of V
stant I
section of this data sheet for more information.
Note that the switch current limit feature does not protect the
circuit if the output is shorted to ground. In this case, current is
only limited by the dc resistance of the inductor and the forward
voltage of the diode.
Inductor Selection—Step-Down Converter
The step-down mode of operation is shown in Figure 15. Unlike
the step-up mode, the ADP1173’s power switch does not
saturate when operating in the step-down mode. Therefore,
switch current should be limited to 650 mA in this mode. If the
input voltage will vary over a wide range, the I
used to limit the maximum switch current. If higher output
current is required, the ADP1111 should be considered.
The first step in selecting the step-down inductor is to calculate
the peak switch current as follows:
where DC = duty cycle (0.55 for the ADP1173)
As previously mentioned, the switch voltage is higher in step-
down mode than step-up mode. V
current and is therefore a function of V
For most applications, a V
The inductor value can now be calculated:
where t
If the input voltage will vary (such as an application that must
operate from a 12 V to 24 V source) an R
selected from Figure 5. The R
rent constant as the input voltage rises. Note that there are separate
R
LIM
V
V
I
V
V
OUT
SW
D
OUT
IN
I
values for step-up and step-down modes of operation.
L
= diode drop (0.5 V for a 1N5818)
PEAK
PEAK
= the minimum input voltage
= voltage drop across the switch
= output current
ON
IN
= the output voltage
V
is at its minimum, then the 1.5 A limit may be ex-
= switch ON time (23 s)
IN
as V
IN(MIN )
. This will improve efficiency by producing a con-
2I
DC
IN
OUT
increases. See the Limiting the Switch Current
I
–V
PEAK
V
SW
IN
PEAK
V
OUT
–V
–V
SW
value calculated for the minimum
OUT
SW
value of 1.5 V is recommended.
V
LIM
D
V
IN
t
ON
resistor will keep switch cur-
D
SW
. In this case, the ADP1173’s
is a function of switch
IN
, L, time and V
LIM
resistor should be
LIM
pin can be
OUT
REV. 0
.
(6)
(7)