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ADP1173
Micropower DC-DC Converter
REV.0
Micropower
DC-DC Converter
FUNCTIONAL BLOCK DIAGRAMS
FEATURES
Operates From 2.0 V to 30 V Input Voltages
Only 110 mA Supply Current (Typical)
Step-Up or Step-Down Mode Operation
Very Few External Components Required
Low Battery Detector On-Chip
User-Adjustable Current Limit
Internal 1 A Power Switch
Fixed or Adjustable Output Voltage Versions
8-Pin DIP or SO-8 Package
APPLICATIONS
Notebook and Palmtop Computers
Cellular Telephones
Flash Memory Vpp Generators
3 V to 5 V, 5 V to 12 V Converters
9 V to 5 V, 12 V to 5 V Converters
Portable Instruments
LCD Bias Generators
GENERAL DESCRIPTIONThe ADP1173 is part of a family of step-up/step-down switching
regulators that operates from an input supply voltage of as little as
2 V to 12 V in step-up mode and to 30 V in step-down mode.
The ADP1173 consumes as little as 110 μA in standby mode,
making it ideal for applications that need low quiescent current.
An auxiliary gain amplifier can serve as a low battery detector,
linear regulator (under voltage lockout) or error amplifier.
The ADP1173 can deliver 80 mA at 5 V from a 3 V input in
step-up configuration or 100 mA at 5 V from a 12 V input in
step-down configuration. For input voltages of less than 2 V use
the ADP1073.
ADP1173–SPECIFICATIONS(@ TA = 08C to +708C, VIN = 3 V unless otherwise noted)OUTPUT SENSE VOLTAGE
CURRENT LIMIT
NOTESThis specification guarantees that both the high and low trip points of the comparator fall within the 1.20 V to 1.30 V range.The output voltage waveform will exhibit a sawtooth shape due to the comparator hysteresis. The output voltage on the fixed output versions will always be within
the specified range.100 kΩ resistor connected between a 5 V source and the AO pin.
ABSOLUTE MAXIMUM RATINGS*Supply Voltage (VIN) . . . . . . . . . . . . . . . . . . . . . . . . . . . .36 V
SW1 Pin Voltage (VSW1) . . . . . . . . . . . . . . . . . . . . . . . . .50 V
SW2 Pin Voltage (VSW2) . . . . . . . . . . . . . . . . . .–0.5 V to VIN
Feedback Pin Voltage (ADP1173) . . . . . . . . . . . . . . . . . . .5 V
Sense Pin Voltage (ADP1173, –3.3, –5, –12) . . . . . . . . .36 V
Maximum Power Dissipation . . . . . . . . . . . . . . . . . .500 mW
Maximum Switch Current . . . . . . . . . . . . . . . . . . . . . . . .1.5 A
Operating Temperature Range . . . . . . . . . . . . .0°C to +70°C
Storage Temperature Range . . . . . . . . . . . . .–65°C to 150°C
Lead Temperature, (Soldering, 10 sec) . . . . . . . . . . . .+300°C
*Stresses above those listed under Absolute Maximum Ratings may cause perma-
nent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those listed in the operational
sections of this specification is not implied. Exposure to absolute maximum
ratings for extended periods of time may affect device reliability.
ORDERING GUIDE*N = Plastic DIP, SO = Small Outline Package.
L1*
100µH
*L1 = COILTRONICS CTX100-4
4X NICAD
ALKALINE
CELLS
IRF7203
+5V
OUTPUT
AT 100mAFigure 1.Step-Up or Step-Down Converter
PIN CONFIGURATIONS
N-8SO-8
8-Lead Plastic DIP8-Lead Plastic SO
CAUTIONESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000V readily
accumulate on the human body and test equipment and can discharge without detection.
Although the ADP1173 features proprietary ESD protection circuitry, permanent damage may
occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD
precautions are recommended to avoid performance degradation or loss of functionality.
PIN FUNCTION DESCRIPTIONS
ADP1173
–Typical Performance Characteristics
SWITCH CURRENT – A
(SAT) – V Figure 2.Saturation Voltage vs.
Switch Current in Step-Up Mode
RLIM – Ω
SWITCH CURRENT – mA
100 Figure 5.Maximum Switch Current
vs. RLIM in Step-Down Mode
INPUT VOLTAGE – Volts
OSCILLATOR FREQUENCY – kHz
24.515 Figure 8.Oscillator Frequency vs.
Input Voltage
SWITCH CURRENT – A
SWITCH ON VOLTAGE – V
0.75 Figure 3.Switch ON Voltage vs.
Switch Current in Step-Down Mode
SWITCH CURRENT – mA
SUPPLY CURRENT – mA
1000100900200300400600700800500 Figure 6.Supply Current vs.
Switch Current
TEMPERATURE – °C–400852570
SET PIN BIAS CURRENT – nAFigure 9.Set Pin Bias Current vs.
Temperature
RLIM – Ω
SWITCH CURRENT – mA
200 Figure 4.Maximum Switch Current
vs. RLIM in Step-Up Mode
Figure 7.Quiescent Current vs.
Temperature
TEMPERATURE – °C–400852570
FEEDBACK PIN BIAS CURRENT – nA
450Figure 10.Feedback Pin Bias Current
vs. Temperature
APPLICATIONS
Theory of OperationThe ADP1173 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 ADP1173 is shown on the
front page. The internal 1.245 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
1.245 V, the 24 kHz oscillator turns on. A driver amplifier pro-
vides base drive to the internal power switch, and the switching
action raises the output voltage. When the voltage at the FB pin
exceeds 1.245 V, the oscillator is shut off. While the oscillator is
off, the ADP1173 quiescent current is only 110 μA. The com-
parator includes a small amount of hysteresis, which ensures
loop stability without requiring external components for fre-
quency compensation.
The maximum current in the internal power switch can be set
by connecting a resistor between VIN and the ILIM pin. When the
maximum current is exceeded, the switch is turned OFF. The
current limit circuitry has a time delay of about 2 μs. If an
external resistor is not used, connect ILIM to VIN. Further
information on ILIM is included in the Limiting the Switch
Current section of this data sheet.
The ADP1173 internal oscillator provides 23 μs ON and 19 μs
OFF times, which is ideal for applications where the ratio
between VIN and VOUT 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 ADP1173 can be connected
as a low battery detector. The inverting input of the gain block
is internally connected to the 1.245 V reference. The noninvert-
ing input is available at the SET pin. A resistor divider, con-
nected between VIN 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 which can sink 100 μA.
The ADP1173 provides external connections for both the
collector and emitter of its internal power switch, which permits
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 VIN.
The output voltage of the ADP1173 is set with two external
resistors. Three fixed-voltage models are also available:
ADP1173-3.3 (+3.3 V), ADP1173-5 (+5 V) and ADP1173-12
(+12 V). The fixed-voltage models are identical to the ADP1173,
except that laser-trimmed voltage-setting resistors are included
on the chip. On the fixed-voltage models of the ADP1173,
simply connect the feedback pin (Pin 8) directly to the output
COMPONENT SELECTION
General Notes on Inductor SelectionWhen the ADP1173 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 then
transferred to the load when the switch turns off. Both the
collector and the emitter of the switch transistor are accessible
on the ADP1173, so the output voltage can be higher, lower or
of opposite polarity than the input voltage.
To specify an inductor for the ADP1173, 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 VIN 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 ADP1173 applications, an inductor ofμH to 470 μH, with a saturation current rating of 300 mA to
1 A and dc resistance <1 Ω is suitable. Ferrite core inductors
which 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 VALUESelecting the proper inductor value is a simple three-step
process:Define the operating parameters: minimum input voltage,
maximum input voltage, output voltage and output current.Select the appropriate conversion topology (step-up, step-
down, or inverting).Calculate the inductor value, using the equations in the
following sections.
Inductor Selection—Step-Up ConverterIn a step-up, or boost, converter (Figure 14), 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: PL=VOUT+VD±VIN(MIN)()×IOUT()(1)
where VD is the diode forward voltage (≈ 0.5 V for a 1N5818
Schottky). Energy is only stored in the inductor while the
ADP1173 switch is ON, so the energy stored in the inductor on
each switching cycle must be must be equal to or greater than:
fOSC(2)
in order for the ADP1173 to regulate the output voltage.
ADP1173When the internal power switch turns ON, current flow in the
inductor increases at the rate of: (t)=VIN1±e
±R′t(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, where the voltage drop across
the switch is small compared to VIN , a simpler equation can be
used: IL(t)=VINt(4)
Replacing “t” in the above equation with the ON time of the
ADP1173 (23 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: EL=1LI2PEAK(5)
As previously mentioned, EL must be greater than PL/fOSC so the
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 generateV at 50 mA from a 3 V source. The inductor power required
is, from Equation 1: PL=(9V+0.5V±3V)×(50mA)=325mW
On each switching cycle, the inductor must supply:
fOSC=325mWkHz=13.5μJ
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: =VIN
IL(MAX)t=3V
500mA23μs=138μH
Substituting a standard inductor value of 100 μH, with 0.2 Ω dc
resistance, will produce a peak switch current of:
IPEAK=3V
1.0Ω1±e
±1.0Ω×23μs
100μH=616mA
Once the peak current is known, the inductor energy can be
calculated from Equation 5: =1(100μH)×(616mA)2=19μJ
The inductor energy of 19 μJ is greater than the PL/fOSC re-
quirement of 13.5 μJ, so the 100 μH inductor will work in this
application. By substituting other inductor values into the same
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 VIN is at its minimum, then the 1.5 A limit may be ex-
ceeded at the maximum value of VIN. In this case, the ADP1173’s
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 IPEAK value calculated for the minimum
value of VIN. This will improve efficiency by producing a con-
stant IPEAK as VIN 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 ConverterThe 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 ILIM pin can be
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:
IPEAK=2IOUT
VOUT+VDIN±VSW+VD(6)
where DC = duty cycle (0.55 for the ADP1173)
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 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: =VIN(MIN)±VSW±VOUT
IPEAK×tON(7)
where tON = switch ON time (23 μs)
If the input voltage will vary (such as an application that must
operate from a 12 V to 24 V source) an RLIM resistor should be
selected from Figure 5. The RLIM resistor will keep switch cur-
rent 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 300 mA is required from a
12 V to +24 V input. Deriving the peak current from Equation 6
yields:
IPEAK=2×300mA
0.55+0.5±1.5+0.5=545mA
The peak current can then be inserted into Equation 7 to calcu-
late the inductor value: =12±1.5±5
545mA×23μs=232μH
Since 232 μH is not a standard value, the next lower standard
value of 220 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 5, a value of 180 Ω will
limit the switch current to 600 mA.
Inductor Selection—Positive-to-Negative ConverterThe configuration for a positive-to-negative converter using the
ADP1173 is shown in Figure 17. 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|+VD()×IOUT()(8)
The ADP1173 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: (t)=VL1±e
_R©t(9)
where R' = 0.65 Ω + RL(DC)
where 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
During each switching cycle, the inductor must supply the
following energy:
fOSC=275mWkHz=11.5μJ
Using a standard inductor value of 220 μH, with 0.2 Ω dc
resistance, will produce a peak switch current of:
IPEAK=4.5V±0.75V
0.65Ω+0.2Ω1±e
±0.85Ω×23μs
220μH=375mA
Once the peak current is known, the inductor energy can be
calculated from Equation 5: EL=1(220μH)×(375mA)2=15.5μJ
The inductor energy of 15.5 μJ is greater than the PL/fOSC
requirement of 11.5 μJ, so the 220 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 to ensure that the peak
current does not exceed 650 mA.
CAPACITOR SELECTIONFor optimum performance, the ADP1173’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
Equivalent Series Inductance (ESL). Low ESR aluminum ca-
pacitors, specifically designed for switch mode converter appli-
cations, 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 11, 12, and 13. These figures show the output
of the same ADP1173 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 11
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
capacitor. The rise in voltage indicates an ESR of about
0.18 Ω. In Figure 12, 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 Ω. Figure 13 shows an OS-CON SA series capacitor in the
same circuit, and ESR is only 0.02 Ω.
*All trademarks are properties of their respective holders.
ADP1173 Figure 11.Aluminum Electrolytic
Figure 12.Tantalum Electrolytic
Figure 13.OS-CON Capacitor
If low output ripple is important, the user should consider the
ADP3000. This device switches at 400 kHz, and the higher
switching frequency simplifies the design of the output filter.
Consult the ADP3000 data sheet for additional details.
All potential current paths must be considered when analyzing
very low power applications, and this includes capacitor leakage
current. OS-CON capacitors have leakage in the 5 μA to 10 μA
range, which will reduce efficiency when the load is also in the
microampere range. Tantalum capacitors, with typical leakage
in the 1 μA to 5 μA range, are recommended for very low power
applications.
DIODE SELECTIONIn specifying a diode, consideration must be given to speed,
forward voltage drop and reverse leakage current. When the
ADP1173 switch turns off, the diode must turn on rapidly if
high efficiency is to be maintained. Schottky rectifiers, as well as
fast signal diodes such as the 1N4148, are appropriate. The
forward voltage of the diode represents power that is not deliv-
ered to the load, so VF must also be minimized. Again, Schottky
diodes are recommended. Leakage current is especially impor-
tant in low current applications, where the leakage can be a
significant percentage of the total quiescent current.
For most circuits, the 1N5818 is a suitable companion to the
ADP1173. This diode has a VF of 0.5 V at 1 A, 4 μA to 10 μA
leakage, and fast turn-on and turn-off times. A surface mount
version, the MBRS130T3, is also available. For applications
where the ADP1173 is “off” most of the time, such as when the
load is intermittent, a silicon diode may provide higher overall
efficiency due to lower leakage. For example, the 1N4933 has a
1 A capability, but with a leakage current of less than 1 μA. The
higher forward voltage of the 1N4933 reduces efficiency when
the ADP1173 delivers power, but the lower leakage may outweigh
the reduction in efficiency.
For switch currents of 100 mA or less, a Schottky diode such as
the BAT85 provides a VF of 0.8 V at 100 mA and leakage less
than 1 μA. A similar device, the BAT54, is available in a SOT23
package. Even lower leakage, in the 1 nA to 5 nA range, can be
obtained with a 1N4148 signal diode.
General purpose rectifiers, such as the 1N4001, are not suitable
for ADP1173 circuits. These devices, which have turn-on times
of 10 μs or more, are too slow for switching power supply
applications. Using such a diode “just to get started” will result
in wasted time and effort. Even if an ADP1173 circuit appears
to function with a 1N4001, the resulting performance will not
be indicative of the circuit performance when the correct diode
is used.
CIRCUIT OPERATION, STEP-UP (BOOST) MODEIn boost mode, the ADP1173 produces an output voltage that
is higher than the input voltage. For example, +12 V can be
generated from a +5 V logic power supply or +5 V can be
derived from two alkaline cells (+3 V).
Figure 16 shows an ADP1173 configured for step-up operation.
The collector of the internal power switch is connected to the
output side of the inductor, while the emitter is connected to
GND. When the switch turns on, pin SW1 is pulled near ground.
This action forces a voltage across L1 equal to VIN–VCE(SAT),
and current begins to flow through L1. This current reaches a
final value (ignoring second-order effects) of: IPEAK≅VIN±VCE(SAT)×23μs
where 23 μs is the ADP1173 switch’s “on” time.
