ISL8560 查看數據表（PDF） - Renesas Electronics

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ISL8560

DC/DC Power Switching Regulator

Renesas Electronics 

ISL8560

The amplitudes of the different types of voltage excursions can

be approximated by using the formulas in Equation 4:

VESR = ESR  Itran

VESL = ESL  -d---I--dt--r--ta----n-

VSAG = -C----o---u-L---t-o---u----t-V-----i-In--t-r--–-a---nV---2-o----u---t--

VHUMP = L-C---o--o-u--u--t--t-----I--tV--r--ao---n-u--2-t-

(EQ. 4)

where

Itran = Output Load Current Transient

Cout = Total Output Capacitance

In a typical converter design, the ESR of the output capacitor

bank dominates the transient response. The ESR and the ESL

are typically the major contributing factors in determining the

output capacitance. The number of output capacitors can be

determined by using Equation 5, which relates the ESR and

ESL of the capacitors to the transient load step and the voltage

limit (Vo):

Number of Caps = --E--------S---------L-----------d--------dt------I----t----r----a--------n----V-+---o--E-----S----R----------I--t--r--a---n--

(EQ. 5)

If VSAG and/or VHUMP are found to be too large for the

output voltage limits, then the amount of capacitance may

need to be increased. In this situation, a trade off between

output inductance and output capacitance may be necessary.

The ESL of the capacitors, which is an important parameter in

Equations 4 and 5, is not usually listed in databooks.

Practically, it can be approximated if an impedance vs

frequency curve is given for a specific capacitor (C):

ESL = C--------2-----------1-------f--r--e---s------2-

(EQ. 6)

where fres is the frequency where the lowest impedance

is achieved (resonant frequency).

The ESL of the capacitors becomes a concern when designing

circuits that supply power to loads with high rates of change in

the current.

Output Inductor Selection

The output inductor is selected to meet the output voltage

ripple requirements and minimize the converter’s response

time to the load transient. The inductor value determines the

converter’s ripple current and the ripple voltage is a function of

the ripple current. The ripple voltage and current are

approximated by Equation 7:

I = VIN - VOUT x VOUT

Fs x L

VIN

VOUT = I x ESR

(EQ. 7)

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values reduce the

FN9244 Rev 7.00

September 19, 2008

converter’s response time to a load transient. Use I of

approximately 30% of IOUT is a good compromise.

One of the parameters limiting the converter’s response to a

load transient is the time required to change the inductor

current. Given a sufficiently fast control loop design, the

ISL8560 will provide either 0% or 100% duty cycle in

response to a load transient. The response time is the time

required to slew the inductor current from an initial current

value to the transient current level. During this interval the

difference between the inductor current and the transient

current level must be supplied by the output capacitor.

Minimizing the response time can minimize the output

capacitance required.

The response time to a transient is different for the application

of load and the removal of load. Equation 8 gives the

approximate response time interval for application and removal

of a transient load:

tRISE =

L x ITRAN

VIN - VOUT

tFALL =

L x ITRAN

VOUT

(EQ. 8)

where: ITRAN is the transient load current step, tRISE is the

response time to the application of load, and tFALL is the

response time to the removal of load. The worst case response

time can be either at the application or removal of load. Be

sure to check both of these equations at the minimum and

maximum output levels for the worst case response time.

Rectifier Selection

Current circulates from ground to the junction of the MOSFET

and the inductor when the high-side switch is off. As a

consequence, the polarity of the switching node is negative

with respect to ground. This voltage is approximately -0.5V (a

Schottky diode drop) during the off time. The rectifier's rated

reverse breakdown voltage must be at least equal to the

maximum input voltage, preferably with a 20% derating factor.

The power dissipation is:

PDW

=

IOUT



VD









1

–

V----V-O---I-U-N---T--

(EQ. 9)

where VD is the voltage of the Schottky diode = 0.5V to 0.7V

Input Capacitor Selection

Use a mix of input bypass capacitors to control the voltage

overshoot across the VIN’s pin. Use small ceramic capacitors

for high frequency decoupling and bulk capacitors to supply the

current needed each time the upper MOSFET turns on. Place

the small ceramic capacitors physically close to the VIN and

PGND pins.

The important parameters for the bulk input capacitance are

the voltage rating and the RMS current rating. For reliable

operation, select bulk capacitors with voltage and current

ratings above the maximum input voltage and largest RMS

current required by the circuit. Their voltage rating should be at

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