ISL8510 查看數據表（PDF） - Renesas Electronics

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ISL8510

Dual Output Controller with 1A Standard Buck PWM and LDO

Renesas Electronics 

ISL8510

There is no upper limit to the output capacitor value. Larger

capacitor values can reduce noise and improve load transient

response, stability and PSRR. The output capacitor should be

located very close to the VOUT pins to minimize impact of PC

board inductances. The other end of the capacitor should be

returned to a clean analog ground.

Buck Regulator Output Capacitor Selection

An output capacitor is required to filter the inductor current and

supply the load transient current. The filtering requirements are

a function of the switching frequency and the ripple current.

The load transient requirements are a function of the slew rate

(di/dt) and the magnitude of the transient load current. These

requirements are generally met with a mix of capacitors and

careful layout.

Embedded processor systems are capable of producing

transient load rates above 1A/ns. High frequency capacitors

initially supply the transient and slow the current load rate seen

by the bulk capacitors. The bulk filter capacitor values are

generally determined by the ESR (Effective Series Resistance)

and voltage rating requirements rather than actual capacitance

requirements.

High frequency decoupling capacitors should be placed as

close to the power pins of the load as physically possible. Be

careful not to add inductance in the circuit board wiring that

could cancel the usefulness of these low inductance

components. Consult with the manufacturer of the load on

specific decoupling requirements.

Use only specialized low-ESR capacitors intended for switching-

regulator applications for the bulk capacitors. The bulk

capacitor’s ESR will determine the output ripple voltage and the

initial voltage drop after a high slew-rate transient. An aluminum

electrolytic capacitor’s ESR value is related to the case size with

lower ESR available in larger case sizes. However, the

Equivalent Series Inductance (ESL) of these capacitors

increases with case size and can reduce the usefulness of the

capacitor to high slew-rate transient loading. Unfortunately, ESL

is not a specified parameter. Work with your capacitor supplier

and measure the capacitor’s impedance with frequency to select

a suitable component. In most cases, multiple electrolytic

capacitors of small case size perform better than a single large

case capacitor.

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 using Equation 3:

I = VIN - VOUT x VOUT

Fs x L

VIN

VOUT = I x ESR (EQ. 3)

Increasing the value of inductance reduces the ripple current

and voltage. However, the large inductance values reduce the

converter’s response time to a load transient. The

recommended I is 30% of the maximum output current.

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

ISL8510 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. Equations 4 and 5 give the

approximate response time interval for application and removal

of a transient load:

tRISE =

L x ITRAN

VIN - VOUT

(EQ. 4)

tFALL =

L x ITRAN

VOUT

(EQ. 5)

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 as shown in Equation 6:

PDW

=

IOUT



VD









1

–

V----V-O---I-U-N---T--

(EQ. 6)

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 MOSFETs. Use small ceramic capacitors

for high frequency decoupling and bulk capacitors to supply the

current needed each time the switching MOSFET turns on.

Place the small ceramic capacitors physically close to the

MOSFET VIN pins (switching MOSFET drain) and the

Schottky diode anode.

The important parameters for the bulk input capacitance are

the voltage rating and the RMS current rating. For reliable

FN6516 Rev 2.00

December 15, 2008

Page 17 of 21

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