2-22

Detailed Component Selection

The application circuits shown in Figure 1 and described by

Tables 1 and 2 illustrate component trade-off to achieve size,

cost and efficiency goals. A design and simulation software

program is available that simplifies the small signal

component selection (http://www.semi.harris.com). This

section provides additional guidance for selecting alternate

components.

Output Capacitor

The output capacitor, C1 smooths the output voltage ripple

of the DC-DC converter. The size and value depend upon

the output ripple requirement, the dielectric characteristics,

the value of output inductance and the switching frequency.

Choose a capacitor with a low impedance at the switching

frequency to meet the output voltage ripple requirement.

Use only specialized low-ESR capacitors intended for

switching-regulator applications.

Capacitor impedance above the switching frequency should

also be minimized. During Hysteretic mode operation, the

transition of RUN from low to high causes inductor current to

ramp from zero to the HMI set level in a very short time. This

rate of current change across the output capacitor’s the

equivalent series inductance (ESL) causes a voltage spike

that appears (attenuated) on the FB pin. The ESL or the rate

of current change must be limited to prevent the hysteretic

comparator from toggling RUN between high and low.

Unfortunately, ESL is not a specified parameter. Work with

your capacitor supplier and measure the capacitor’s

impedance at the switching frequency (and the first few

harmonics of the switching 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

The output inductor, L1 sets the ripple current and influences

the converter efficiency. The ripple current,

∆I is related to

inductor current. Increasing the inductance or the switching

frequency lowers the ripple current and the output ripple volt-

age. The inductance can be determined by:

Inductance is a function of the core permeability, core size,

and the square of number of turns. The power dissipation of

the inductor is also dependent upon the number of turns and

the core. In general, most of the power dissipation is in the

inductor’s winding. Therefore, use high permeability core

material to minimize the number of turns. Be sure the flux at

full load current does not saturate the core. Recommended

core materials include: Microlite™ from Allied Signal, ferrite,

Kool-Mu™, molypermalloy (MMP), and powdered iron.

Switching Frequency

The oscillator produces a sawtooth wave on the CT pin with

an amplitude of 1.26V. The switching frequency is set by C6.

Select the closest standard capacitance value according to

the following formula:

Higher switching frequency decreases the size of output

filter L1 and C1 and enables a higher bandwidth converter

for faster response to a load transient. However, higher

frequencies dissipate more power for a less efficient

converter.

Control Loop Design

The HIP5020 realizes excellent transient response with

proper control loop design. The device utilizes peak-current

control with the entire current loop integrated within the

HIP5020. Additionally, the HIP5020 includes a 12pF

integration capacitor across the error amplifier. (See the

Detailed Operating Description above.) Some applications

need only add the resister R1 and capacitor C7 for a

complete design.

The capacitor, C7 adds a compensation slope to the peak

current control loop (see Slope Compensation below). C7

shows up in the closed loop transfer function as peaking

around half of the switching frequency. For a stable design,

make sure the closed loop gain at half of the switching

frequency is below -10dB.

The error amplifier and compensation components regulate

the output voltage by controlling the current loop (as shown

in Figure 5). The compensation components shown in

Figure 8 realize a lead-lag circuit. The resistor R1 adjusts

the loop gain of the converter and resistor R6 and capacitor

C9 set the pole and zero. The resistor R2 does not appear in

the lead-lag transfer function. R2 sets the output voltage

level. First stabilize control loop by selecting R1 and then

determine R2 for the desired output voltage level.

L1

V

–

O

∆IF

S

•

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

V

O

V

IN

---------

•

=

C6

10

4

–

F

S

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

10

11

–

–

=

REFERENCE

1.26V

R1

R2

TO

PWM

COMPARATOR

12pF

R6

C9

FB

FIGURE 8. LEAD-LAG COMPENSATION CIRCUIT

ERROR

-

+

AMP

HIP5020

HIP5020