Capacitors with a Y5V or Z5U temperature characteristic are

generally not recommended for use with the LM2797/98.

These types of capacitors typically have wide capacitance

tolerance (+80%, -20%) and vary significantly over tempera-

ditions, a 1µF-rated Y5V or Z5U capacitor could have a

capacitance as low as 0.1µF. Such detrimental deviation is

likely to cause these Y5V and Z5U capacitors to fail to meet

the minimum capacitance requirements of the LM2797/98.

The table below lists some leading ceramic capacitor manu-

facturers.

Manufacturer

Contact Information

AVX

www.avx.com

Murata

www.murata.com

Taiyo-Yuden

www.t-yuden.com

TDK

www.component.tdk.com

Vishay-Vitramon

www.vishay.com

OUTPUT CAPACITOR

The output capacitor of the LM2797/98 greatly affect perfor-

mance of the circuit. In typical high-current applications, a

10µF low-ESR (ESR = equivalent series resistance) ceramic

capacitor is recommended. For lighter loads, the output

capacitance may be reduced (as low as 1µF for output

currents

≤ 60mA is usually acceptable). The performance of

the part should be evaluated with special attention paid to

efficiency and output ripple to ensure the capacitance cho-

sen on the output yields performance suitable for the appli-

cation. In extreme cases, excessive ripple could cause con-

trol loop instability, severely affecting the performance of the

part. If excessive ripple is present, the output capacitance

should be increased.

The ESR of the output capacitor affects charge pump output

resistance, which plays a role in determining output current

capability. Both output capacitance and ESR affect output

voltage ripple (See Output Voltage Ripple section, above).

For these reasons, a low-ESR X7R/X5R ceramic capacitor is

the capacitor of choice for the LM2797/98 output.

FLYING CAPACITORS

The flying capacitors (C

input to the output, and determine the strength of the charge

pump: the larger the capacitance, the greater the output

current capability. If capacitors are too small, the LM2797/98

could spend excessive amount of time gain hopping: de-

creasing efficiency, increasing output voltage ripple, and

possibly impeding the ability of the part to regulate. On the

other hand, if the flying capacitors are too large they could

potentially overwhelm the output capacitor, resulting in in-

creased output voltage ripple.

Low-ESR ceramic capacitors with X7R or X5R temperature

characteristic are strongly recommended for use here. The

flying capacitors C1 and C2 should be identical. As a general

rule, the capacitance value of each flying capacitor should

be 1/10th that of the output capacitor. ESR should be as low

as possible to minimize the output resistance of the charge

pump and give maximum output current capability. Polarized

capacitor (tantalum, aluminum electrolytic, etc.) must not be

used for the flying capacitors, as they could become reverse-

biased upon start-up of the LM2797/98.

INPUT CAPACITOR

The input capacitor (C

quick transfer of charge from the supply to the flying capaci-

tors during the charge phase of operation. The input capaci-

tor helps to keep the input voltage from drooping at the start

of the charge phase when the flying capacitor is connected

to the input, and helps to filter noise on the input pin that

could adversely affect sensitive internal analog circuitry bi-

ased off the input line. An X7R/X5R ceramic capacitor is

recommended for use. As a general recommendation, the

input capacitor should be chosen to match the output capaci-

tor.

POWER DISSIPATION

LM2797/98 power dissipation will, typically, not be much of a

concern in most applications. Derating to accommodate self-

heating will rarely be required due to the high efficiency of

the part. Peak power dissipation (P

tions is seen with the LM2798-1.5 operating at V

and I

ratings). Under these conditions, the power efficiency (E) of

the LM2798-1.5 is 54% (typ.). Assuming a typical junction-

to-ambient thermal resistance (

θ

of 220˚C/Watt, the junction temperature (T

calculated below for a part operating at the maximum rated

ambient temperature (T

P

=(P

= [(1/E) - 1] x P

OUT

= [(1/64%) - 1] x 1.5V x 120mW

= 153mW

T

θ

= 85˚C + (.153W x 220˚C/W)

=119˚C

Even under these peak power dissipation and ambient tem-

perature conditions, the junction temperature of the LM2798-

1.5 is below the maximum operating rating of 125˚C.

As an additional note, the ambient temperature operating

rating range listed in the specifications is provided merely for

convenience. The LM2797/98 may be operated outside this

rating, so long as the junction temperature of the device

does not exceed the maximum operating rating of 125˚C.

www.national.com

12