7

∆I is the output inductor ripple current.

For an equation for the ripple current see the section under

component guidelines titled ‘Output Inductor Selection’.

Output Voltage Selection

The output voltage of the PWM converter can be resistor-

the compensation components, it is advisable its value is

kept between 2k

Ω and 5kΩ.

All linear regulators’ output voltages are set by means of

external resistor dividers as shown in Figure 4. The two

resistors used to set the voltage on each of the three linear

regulators have to meet the following criteria: their value

while in a parallel connection has to be less than 5k

Ω, or

otherwise said, the following relationship has to be met:

There may be a second restriction on the size of the

resistors used to set the linear regulators’ output voltage

based on ACPI functionality. Read the ‘ACPI

Implementation’ section under ‘Application Guidelines’ to

see if this additional constraint concerns your application. To

ensure the parallel combination of the feedback resistors

equations:

, where

Application Guidelines

Soft-Start Interval

The soft-start function controls the output voltages rate of rise

to limit the current surge at start-up. The soft-start function is

integrated on the chip and the soft-start interval is thus fixed.

Layout Considerations

MOSFETs switch very fast and efficiently. The speed with

which the current transitions from one device to another

causes voltage spikes across the interconnecting

impedances and parasitic circuit elements. The voltage

spikes can degrade efficiency, radiate noise into the circuit,

and lead to device overvoltage stress. Careful component

layout and printed circuit design minimizes the voltage

spikes in the converter. Consider, as an example, the turn-

off transition of the upper PWM MOSFET. Prior to turn-off,

the upper MOSFET was carrying the full load current.

During the turn-off, current stops flowing in the upper

MOSFET and is picked up by the lower MOSFET or

Schottky diode. Any inductance in the switched current

path generates a large voltage spike during the switching

interval. Careful component selection, tight layout of the

critical components, and short, wide circuit traces minimize

the magnitude of voltage spikes. See the Application Note

AN9908 for evaluation board drawings of the component

placement and printed circuit board.

There are two sets of critical components in a DC-DC

converter using a HIP6521 controller. The switching power

components are the most critical because they switch large

amounts of energy, and as such, they tend to generate

UGATE

OCSET

PHASE

OCC

+

-

GATE

CONTROL

VCC

OC

40

µA

OVERCURRENT TRIP:

+

+

FIGURE 3. OVERCURRENT DETECTION

PWM

DRIVE

i

D

r

DS ON

()

×

I

OCSET

R

OCSET

×

>

V

DS

V

SET

>

V

PHASE

V

IN

V

DS

–

=

V

OCSET

V

IN

V

SET

–

=

DRIVE3

FB3

FB4

Q4

HIP6521

Q5

DRIVE4

V

OUT

0.8

1

R

S

R

P

--------

+







×

=

FIGURE 4. ADJUSTING THE OUTPUT VOLTAGE OF ANY OF

THE FOUR REGULATORS (OUTPUTS 3 AND 4

PICTURED)

+

+

R

S

R

P

×

R

S

R

P

+

----------------------

5k

Ω

<

R

S

V

OUT

V

FB

----------------

R

FB

×

=

R

P

R

S

V

FB

×

V

OUT

V

FB

–

---------------------------------

=

HIP6521