HIP6007

FN4307 Rev.1.00

Page 8 of 10

September 1997

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 by the following equations:

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.

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 HIP6007 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. The following equations give

the approximate response time interval for application and

removal of a transient load:

response time to the removal of load. With a +5V input source,

the worst case response time can be either at the application

or removal of load and dependent upon the output voltage

setting. Be sure to check both of these equations at the

minimum and maximum output levels for the worst case

response time.

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 Q1 turns on. Place the small ceramic

capacitors physically close to the MOSFETs and between the

drain of Q1 and the anode of Schottky diode D2.

The important parameters for the bulk input capacitor are the

voltage rating and the RMS current rating. For reliable

operation, select the bulk capacitor with voltage and current

ratings above the maximum input voltage and largest RMS

current required by the circuit. The capacitor voltage rating

should be at least 1.25 times greater than the maximum input

voltage and a voltage rating of 1.5 times is a conservative

guideline. The RMS current rating requirement for the input

capacitor of a buck regulator is approximately 1/2 the DC load

current.

For a through hole design, several electrolytic capacitors

(Panasonic HFQ series or Nichicon PL series or Sanyo MV-GX

or equivalent) may be needed. For surface mount designs,

solid tantalum capacitors can be used, but caution must be

exercised with regard to the capacitor surge current rating.

These capacitors must be capable of handling the surge-

current at power-up. The TPS series available from AVX, and

the 593D series from Sprague are both surge current tested.

MOSFET Selection/Considerations

The HIP6007 requires an N-Channel power MOSFET. It should

and thermal management requirements.

In high-current applications, the MOSFET power dissipation,

package selection and heatsink are the dominant design

factors. The power dissipation includes two loss components;

conduction loss and switching loss. The conduction losses

are the largest component of power dissipation for the

MOSFET. Switching losses also contribute to the overall

MOSFET power loss (see the equations below). These

equations assume linear voltage-current transitions and are

approximations. The gate-charge losses are dissipated by the

HIP6007 and don't heat the MOSFET. However, large gate-

the upper MOSFET switching losses. Ensure that the

MOSFET is within its maximum junction temperature at high

ambient temperature by calculating the temperature rise

according to package thermal-resistance specifications. A

separate heatsink may be necessary depending upon

MOSFET power, package type, ambient temperature and air

flow.

Standard-gate MOSFETs are normally recommended for use

with the HIP6007. However, logic-level gate MOSFETs can be

used under special circumstances. The input voltage, upper

gate drive level, and the MOSFET’s absolute gate-to-source

voltage rating determine whether logic-level MOSFETs are

appropriate.

I =



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

V

1

Fs is the switching frequency.