ADP1111 datasheet(8/16 Pages) AD

8 / 16 page

ADP1111

–8–

REV. 0

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:

IN MIN

() −VSW −VOUT

IPEAK

• t

(Equation 7)

where tON = 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 RLIM resistor

should be selected from Figure 6. 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 300 mA is required from a

+12 V to +24 V source. Deriving the peak current from

Equation 6 yields:

IPEAK =

• 300 mA

0.5

+ 0.5

−1.5 + 0.5









= 600 mA

Then, the peak current can be inserted into Equation 7 to

calculate the inductor value:

−1.5 − 5

600 mA

• 7µs = 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 RLIM 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:

OUT

()•()

(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:

IL t

() = VL

− e

−R't









(Equation 9)

where: R' = 0.65

Ω + R

L(DC)

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:

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:

IPEAK =

4.5V

− 0.75V

0.65

Ω+0.2 Ω

− e

−0.85Ω• 7 µs

µH











 = 445 mA

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

calculated from (Equation 9):

EL =

µH

()• 445mA

()2 =5.54 µJ

Since the inductor energy of 5.54

µJ is greater than the P

L/fOSC

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 RLIM resistor and the ILIM pin can be connected

directly to VIN. 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.

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