SP6123 データシートの表示（PDF） - Signal Processing Technologies

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SP6123

Low Voltage, Synchronous Step-Down PWM Controller

Signal Processing Technologies 

the value corresponding to the new load current.

Additionally, the ESR in the output capacitor

causes a step in the output voltage equal to the

ESR value multiplied by the change in load

current. Because of the fast transient response

provided by the SP6123 when exposed to output

load transient, the output capacitor is typically

chosen for ESR , not for capacitance value.

The output capacitor’s ESR, combined with the

inductor ripple current, is typically the main con-

tributor to output voltage ripple. The maximum

allowable ESR required to maintain a specified

output voltage ripple can be calculated by:

RESR

≤

∆VOUT

I PP

where:

∆VOUT = peak to peak output voltage ripple

IPP = peak to peak inductor ripple current

The total output ripple is a combination of the

ESR and the output capacitance value and can

be calculated as follows:

( ) ∆VOUT =

IPP (1 – D)

COUTFS

2 + (IPPRESR)2

where:

D = duty cycle equal to VOUT/VIN

COUT = output capacitance value

Recommended capacitors that can be used ef-

fectively in SP6123 applications are: low-ESR

aluminum electrolytic capacitors, OS-CON ca-

pacitors that provide a very high performance/

size ratio for electrolytic capacitors and low-

ESR tantalum capacitors. AVX TPS series and

Kemet T510 surface mount capacitors are popu-

lar tantalum capacitors that work well in SP6123

applications. POSCAP from Sanyo is a solid

electrolytic chip capacitor that has low ESR and

high capacitance. For the same ESR value,

POSCAP has lower profile compared with tan-

talum capacitor.

Panasonic offers the SP series of specialty poly-

mer aluminum electrolytic surface mount ca-

APPLICATIONS INFORMATION

pacitors. These capacitors have a lower ESR

than tantalum capacitors, reducing the total num-

ber of capacitance required for a given transient

response.

Input Capacitor Selection

The input capacitor should be selected for ripple

current rating, capacitance and voltage rating.

The input capacitor must meet the ripple current

requirement imposed by the switching current.

In continuous conduction mode, the source cur-

rent of the high-side MOSFET is approximately

a square wave of duty cycle VOUT/ VIN. Most of

this current is supplied by the input bypass

capacitors. The RMS value of input capacitor

current is determined at the maximum output

current and under the assumption that the peak

to peak inductor ripple current is low, it is given

by:

ICIN(rms) = IOUT(max) √D(1 - D)

The worse case occurs when the duty cycle, D,

is 50% and gives an RMS current value equal to

IOUT/2. Select input capacitors with adequate

ripple current rating to ensure reliable opera-

tion.

The power dissipated in the input capacitor is:

P = I R 2

C IN

CIN ( rms) ESR( CIN)

This can become a significant part of power

losses in a converter and hurt the overall energy

transfer efficiency.

The input voltage ripple primarily depends on

the input capacitor ESR and capacitance. Ignor-

ing the inductor ripple current, the input voltage

ripple can be determined by:

∆ VIN

= I out(max)

RE SR(CIN )

+

I OUT (MAX V) OUT (VIN

FSC INVIN 2

−VOUT )

The capacitor type suitable for the output ca-

pacitors can also be used for the input capaci-

tors. However, exercise extra caution when tan-

talum capacitors are considered. Tantalum ca-

pacitors are known for catastrophic failure when

exposed to surge current, and input capacitors

are prone to such surge current when power

supplies are connected ‘live’ to low impedance

Date: 5/25/04

SP6123 Low Voltage, Synchronous Step Down PWM Controller

10

© Copyright 2004 Sipex Corporation

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