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ADP1111ARZ-3.3

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
ADP1111
–6–
REV. 0
THEORY OF OPERATION
The ADP1111 is a flexible, low-power, switch-mode power
supply (SMPS) controller. The regulated output voltage can be
greater than the input voltage (boost or step-up mode) or less
than the input (buck or step-down mode). This device uses a
gated-oscillator technique to provide very high performance
with low quiescent current.
A functional block diagram of the ADP1111 is shown on
the first page of this data sheet. The internal 1.25 V reference is
connected to one input of the comparator, while the other input
is externally connected (via the FB pin) to a feedback network
connected to the regulated output. When the voltage at the FB
pin falls below 1.25 V, the 72 kHz oscillator turns on. A driver
amplifier provides base drive to the internal power switch, and
the switching action raises the output voltage. When the voltage
at the FB pin exceeds 1.25 V, the oscillator is shut off. While
the oscillator is off, the ADP1111 quiescent current is only
300 μA. The comparator includes a small amount of hysteresis,
which ensures loop stability without requiring external compo-
nents for frequency compensation.
The maximum current in the internal power switch can be set
by connecting a resistor between V
IN
and the I
LIM
pin. When the
maximum current is exceeded, the switch is turned OFF. The
current limit circuitry has a time delay of about 1 μs. If an
external resistor is not used, connect I
LIM
to V
IN
. Further
information on I
LIM
is included in the “APPLICATIONS”
section of this data sheet.
The ADP1111 internal oscillator provides 7 μs ON and 7 μs
OFF times that are ideal for applications where the ratio
between V
IN
and V
OUT
is roughly a factor of two (such as
converting +3 V to + 5 V). However, wider range conversions
(such as generating +12 V from a +5 V supply) can easily be
accomplished.
An uncommitted gain block on the ADP1111 can be connected
as a low-battery detector. The inverting input of the gain block
is internally connected to the 1.25 V reference. The noninverting
input is available at the SET pin. A resistor divider, connected
between V
IN
and GND with the junction connected to the SET
pin, causes the AO output to go LOW when the low battery set
point is exceeded. The AO output is an open collector NPN
transistor that can sink 300 μA.
The ADP1111 provides external connections for both the
collector and emitter of its internal power switch that permit
both step-up and step-down modes of operation. For the step-
up mode, the emitter (Pin SW2) is connected to GND, and the
collector (Pin SW1) drives the inductor. For step-down mode,
the emitter drives the inductor while the collector is connected
to V
IN
.
The output voltage of the ADP1111 is set with two external
resistors. Three fixed-voltage models are also available:
ADP1111–3.3 (+3.3 V), ADP1111–5 (+5 V) and ADP1111–12
(+12 V). The fixed-voltage models are identical to the
ADP1111, except that laser-trimmed voltage-setting resistors
are included on the chip. On the fixed-voltage models of the
ADP1111, simply connect the feedback pin (Pin 8) directly to
the output voltage.
COMPONENT SELECTION
General Notes on Inductor Selection
When the ADP1111 internal power switch turns on, current
begins to flow in the inductor. Energy is stored in the inductor
core while the switch is on, and this stored energy is transferred
to the load when the switch turns off. Since both the collector
and the emitter of the switch transistor are accessible on the
ADP1111, the output voltage can be higher, lower, or of
opposite polarity than the input voltage.
To specify an inductor for the ADP1111, the proper values of
inductance, saturation current and dc resistance must be
determined. This process is not difficult, and specific equations
for each circuit configuration are provided in this data sheet. In
general terms, however, the inductance value must be low
enough to store the required amount of energy (when both
input voltage and switch ON time are at a minimum) but high
enough that the inductor will not saturate when both V
IN
and
switch ON time are at their maximum values. The inductor
must also store enough energy to supply the load, without
saturating. Finally, the dc resistance of the inductor should be
low so that excessive power will not be wasted by heating the
windings. For most ADP1111 applications, an inductor of
15 μH to 100 μH with a saturation current rating of 300 mA to
1 A and dc resistance <0.4 Ω is suitable. Ferrite-core inductors
that meet these specifications are available in small, surface-
mount packages.
To minimize Electro-Magnetic Interference (EMI), a toroid or
pot-core type inductor is recommended. Rod-core inductors are
a lower-cost alternative if EMI is not a problem.
CALCULATING THE INDUCTOR VALUE
Selecting the proper inductor value is a simple three step
process:
1. Define the operating parameters: minimum input voltage,
maximum input voltage, output voltage and output current.
2. Select the appropriate conversion topology (step-up, step-
down, or inverting).
3. Calculate the inductor value using the equations in the
following sections.
TEMPERATURE – ⴗC
350
300
0
–40 8525
200
150
100
50
250
BIAS CURRENT
BIAS CURRENT – μA
070
Figure 14. Set Pin Bias Current vs. Temperature
REV. A
ADP1111
–7–
REV. 0
INDUCTOR SELECTION–STEP-UP CONVERTER
In a step-up or boost converter (Figure 18), the inductor must
store enough power to make up the difference between the input
voltage and the output voltage. The power that must be stored
is calculated from the equation:
P
L
= V
OUT
+V
D
−V
IN(MIN )
()
• I
OUT
()
(Equation 1)
where V
D
is the diode forward voltage (0.5 V for a 1N5818
Schottky). Because energy is only stored in the inductor while
the ADP1111 switch is ON, the energy stored in the inductor
on each switching cycle must be equal to or greater than:
P
f
L
OSC
(Equation 2)
in order for the ADP1111 to regulate the output voltage.
When the internal power switch turns ON, current flow in the
inductor increases at the rate of:
I
L
t
()
=
V
IN
R'
1− e
−R't
L
⎛
⎝
⎜
⎞
⎠
⎟
(Equation 3)
where L is in Henrys and R' is the sum of the switch equivalent
resistance (typically 0.8 Ω at +25°C) and the dc resistance of
the inductor. In most applications, the voltage drop across the
switch is small compared to V
IN
so a simpler equation can be
used:
I
L
t
()
=
V
IN
L
t
(Equation 4)
Replacing ‘t’ in the above equation with the ON time of the
ADP1111 (7 μs, 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:
E
L
=
1
2
L • I
2
PEAK
(Equation 5)
As previously mentioned, E
L
must be greater than P
L
/f
OSC
so
that the ADP1111 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 possible to minimize losses in the switch, inductor and
diode.
In practice, the inductor value is easily selected using the
equations above. For example, consider a supply that will
generate 12 V at 40 mA from a 9 V battery, assuming a 6 V
end-of-life voltage. The inductor power required is, from
Equation 1:
P
L
= 12V +0.5V −6V
()
• 40 mA
()
= 260 mW
On each switching cycle, the inductor must supply:
P
L
f
OSC
=
260 mW
72 kHz
= 3.6 μJ
Since the required inductor power is fairly low in this example,
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:
L =
V
IN
I
L(MAX )
t =
6V
500 mA
7 μs = 84 μH
Substituting a standard inductor value of 68 μH with 0.2 Ω dc
resistance will produce a peak switch current of:
I
PEAK
=
6V
1. 0 Ω
1− e
−1. 0 Ω•7 μs
68 μH
⎛
⎝
⎜
⎞
⎠
⎟
= 587 mA
Once the peak current is known, the inductor energy can be
calculated from Equation 5:
E
L
=
1
2
68 μH
()
• 587 mA
()
2
=11.7 μJ
Since the inductor energy of 11.7 μJ is greater than the P
L
/f
OSC
requirement of 3.6 μJ, the 68 μH inductor will work in this
application. By substituting other inductor values into the same
equations, the optimum inductor value can be selected.
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 ADP1110 should be
considered. Since this device has a 70% duty cycle, more energy
is stored in the inductor on each cycle. This results is 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
IN
is at its minimum, the 1.5 A limit may be exceeded at
the maximum value of V
IN
. In this case, the ADP1111’s current
limit feature can be used to limit switch current. Simply select a
resistor (using Figure 6) that will limit the maximum switch
current to the I
PEAK
value calculated for the minimum value of
V
IN
. This will improve efficiency by producing a constant I
PEAK
as V
IN
increases. See the “Limiting the Switch Current” 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 19.
Unlike the step-up mode, the ADP1111’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
LIM
pin can be
used to limit the maximum switch current. Higher switch
current is possible by adding an external switching transistor as
shown in Figure 21.
The first step in selecting the step-down inductor is to calculate
the peak switch current as follows:
I
PEAK
=
2 I
OUT
DC
V
OUT
+ V
D
V
IN
−V
SW
+V
D
⎛
⎝
⎜
⎞
⎠
⎟
(Equation 6)
where DC = duty cycle (0.5 for the ADP1111)
V
SW
= voltage drop across the switch
V
D
= diode drop (0.5 V for a 1N5818)
I
OUT
= output current
V
OUT
= the output voltage
V
IN
= the minimum input voltage
REV. A
ADP1111
–8–
REV. 0
As previously mentioned, the switch voltage is higher in step-
down mode than in step-up mode. V
SW
is a function of switch
current and is therefore a function of V
IN
, L, time and V
OUT
.
For most applications, a V
SW
value of 1.5 V is recommended.
The inductor value can now be calculated:
L =
V
IN MIN
()
−V
SW
−V
OUT
I
PEAK
•t
ON
(Equation 7)
where t
ON
= switch ON time (7 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 R
LIM
resistor
should be selected from Figure 6. The R
LIM
resistor will keep
switch current constant as the input voltage rises. Note that
there are separate R
LIM
values for step-up and step-down modes
of operation.
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:
I
PEAK
=
2 •300 mA
0.5
5 + 0.5
12 −1. 5 + 0.5
⎛
⎝
⎜
⎞
⎠
⎟
= 600 mA
Then, the peak current can be inserted into Equation 7 to
calculate the inductor value:
L =
12 −1. 5 − 5
600 mA
• 7 μs = 64 μH
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 R
LIM
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.
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:
P = I
L OUT
VV
OUT D
+
()
•
()
(Equation 8)
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:
I
L
t
()
=
V
L
R'
1 − e
−R't
L
⎛
⎝
⎜
⎞
⎠
⎟
(Equation 9)
where: R' = 0.65 Ω + R
L(DC)
V
L
= V
IN
– 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:
P
L
= |−5V|+0.5V|
()
• 50 mA
()
= 275 mW
During each switching cycle, the inductor must supply the
following energy:
P
L
f
OSC
=
275 mW
72 kHz
= 3.8 μJ
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 − 0.75 V
0.65 Ω+0.2 Ω
1− e
−0.85 Ω•7μs
56 μH
⎛
⎝
⎜
⎞
⎠
⎟
= 445 mA
Once the peak current is known, the inductor energy can be
calculated from (Equation 9):
E
L
=
1
2
56 μH
()
• 445 mA
()
2
= 5.54 μJ
Since the inductor energy of 5.54
μ
J is greater than the P
L
/f
OSC
requirement of 3.82 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
application. Therefore, the peak current will not change enough
to require an R
LIM
resistor and the I
LIM
pin can be connected
directly to V
IN
. Care should be taken, of course, to ensure that
the peak current does not exceed 650 mA.
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.
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.
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.
REV. A
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

ADP1111ARZ-3.3

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Switching Voltage Regulators Micropwr Adj & Fixed 3.3V
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