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

HIP6004B 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

DACOUT setting. Be sure to check both of these equations

at the minimum and maximum output levels for the worst

case response time. With a +12V input, and output voltage

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

small ceramic capacitors physically close to the MOSFETs

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 HIP6004B requires 2 N-Channel power MOSFETs. These

requirements, 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 both the upper

and the lower MOSFETs. These losses are distributed between

the two MOSFETs according to duty factor (see the equations

below). Only the upper MOSFET has switching losses, since

the Schottky rectifier clamps the switching node before the

synchronous rectifier turns on. These equations assume linear

voltage-current transitions and do not adequately model power

loss due the reverse-recovery of the lower MOSFET’s body

diode. The gate-charge losses are dissipated by the HIP6004B

and don't heat the MOSFETs. However, large gate-charge

MOSFET switching losses. Ensure that both MOSFETs are

within their 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 HIP6004B. 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.

Figure 9 shows the upper gate drive (BOOT pin) supplied by a

develops a floating supply voltage referenced to the PHASE

turns on. Logic-level MOSFETs can only be used if the

MOSFET’s absolute gate-to-source voltage rating exceeds

1

2

HIP6004B