Micropower, Step-Up/Step-Down Switching

a Regulator; Adjustable and Fixed 3.3 V, 5 V, 12 V

ADP1110

FEATURES

Operates at Supply Voltages From 1.0 V to 30 V

Step-Up or Step-Down Mode

Minimal External Components Required

Low-Battery Detector

User-Adjustable Current Limiting

Fixed or Adjustable Output Voltage Versions

8-Pin DIP or SO-8 Package

APPLICATIONS

Cellular Telephones

Single-Cell to 5 V Converters

Laptop and Palmtop Computers

Pagers

Cameras

Battery Backup Supplies

Portable Instruments

Laser Diode Drivers

Hand-Held Inventory Computers

FUNCTIONAL BLOCK DIAGRAMS

SET

ADP1110

A2

VIN

GAIN BLOCK/

ERROR AMP

220mV

REFERENCE

A1 OSCILLATOR

A0

ILIM

SW1

Q1

COMPARATOR

R2

R1 300kΩ

DRIVER

GND

SENSE

SW2

ADP1110 Block Diagram—Fixed Output Version

SET

ADP1110

GENERAL DESCRIPTION

The ADP1110 is part of a family of step-up/step-down switch-

ing regulators that operate from an input voltage supply as little

as 1.0 V. This very low input voltage allows the ADP1110 to be

used in applications that use a single cell as the primary power

source.

The ADP1110 can be configured to operate in either step-up or

step-down mode, but for input voltages greater than 3 V, the

ADP1111 would be a more effective solution.

An auxiliary gain amplifier can serve as a low battery detector or

as a linear regulator.

The quiescent current of 300 µA makes the ADP1110 useful in

remote or battery powered applications.

A2

VIN

GAIN BLOCK/

ERROR AMP

220mV

REFERENCE

A1 OSCILLATOR

A0

ILIM

SW1

Q1

COMPARATOR

DRIVER

GND

FB

SW2

ADP1110 Block Diagram—Adjustable Output Version

The 70 kHz frequency operation also allows for the use of

surface-mount external capacitors and inductors.

Battery protection circuitry limits the effect of reverse current to

safe levels at reverse voltages up to 1.6 V.

REV. 0

Information furnished by Analog Devices is believed to be accurate and

reliable. However, no responsibility is assumed by Analog Devices for its

use, nor for any infringements of patents or other rights of third parties

which may result from its use. No license is granted by implication or

otherwise under any patent or patent rights of Analog Devices.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.

Tel: 617/329-4700 World Wide Web Site: http://www.analog.com

Fax: 617/326-8703

© Analog Devices, Inc., 1996

ADP1110

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.

INDUCTOR SELECTION–STEP-UP CONVERTER

In a step-up or boost converter (Figure 19), 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 −V IN(MIN ) • IOUT

(Equation 1)

where VD is the diode forward voltage (≈ 0.5 V for a 1N5818

Schottky). Because energy is only stored in the inductor while

the ADP1110 switch is ON, the energy stored in the inductor

on each switching cycle must be must be equal to or greater

than:

PL

fOSC (Equation 2)

in order for the ADP1110 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. If the voltage drop across the switch is small

compared to VIN, a simpler equation can be used:

IL

(t

)

= V IN

L

t

(Equation 4)

Replacing ‘t’ in the above equation with the ON time of the

ADP1110 (10 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

=

1

L

2

•

I

2PEAK

(Equation 5)

As previously mentioned, EL must be greater than PL/fOSC so

that the ADP1110 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

120 mA from a 4.5 V to 8 V source. The inductor power required

is from Equation 1:

( )PL = 12 V + 0.5 V − 4.5 V •120 mA = 960 mW

On each switching cycle, the inductor must supply:

PL

f OSC

=

960 mW

70 kHz

=13.7 µJ

Assuming a peak current of 1 A as a starting point, (Equation 4)

can be rearranged to recommend an inductor value:

L = V IN t = 4.5V 10 µs = 45 µH

IL(MAX ) 1 A

Substituting a standard inductor value of 47 µH with 0.2 Ω dc

resistance will produce a peak switch current of:

I PEAK

=

4.5V

1.0 Ω

–1.0 Ω •10 µs

1− e 47 µH

= 862 mA

Once the peak current is known, the inductor energy can be

calculated from Equation 5:

EL

=

1

2

(47

µH

)

•

(862

mA)2

=

17.5

µJ

Since the inductor energy of 17.5 µJ is greater than the PL/fOSC

requirement of 13.7 µJ, the 47 µH inductor will work in this

application. By substituting other inductor values into the same

equations, the optimum inductor value can be determined.

When selecting an inductor, the peak current must not exceed

the maximum switch current of 1.5 A.

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, the 1.5 A limit may be exceeded at the

maximum value of VIN. In this case, the ADP1110’s current limit

feature can be used to limit switch current. Simply select a resistor

(using Figure 7) 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 constant 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 CONVERTER

The step-down mode of operation is shown in Figure 20.

Unlike the step-up mode, the ADP1110’s power switch does not

saturate when operating in the step-down mode; therefore,

switch current should be limited to 800 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. Higher switch

current is possible by adding an external switching transistor as

shown in Figure 22.

The first step in selecting the step-down inductor is to calculate

the peak switch current as follows:

IPEAK

= 2 IOUT

DC

V

V

IN

OUT +VD

–V SW +V

D

(Equation 6)

where: DC = duty cycle (0.69 for the ADP1110)

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

REV. 0

–7–