uP1537

12

uP1537-DS-F0000, Feb. 2018

www.upi-semi.com

Application Information

Output Inductor Selection

The inductor plays an important role in step-down

converters because it stores the energy from the input

power rail and then releases the energy to the load. From

the viewpoint of efficiency, the dc resistance (DCR) of the

inductor should be as small as possible to minimize the

conduction loss. In addition, the inductor covers a

significant proportion of the board space, so its size is

also important. Low profile inductors can save board space

especially when the height has a limitation. However, low

DCR and low profile inductors usually cost ineffective.

Additionally, larger inductance results in lower ripple

current which translates into the lower power loss.

However, the inductor current rising time increases with

inductance value. This means the transient response will

be slower. Therefore, the inductor design is a trade-off

between performance, size and cost.

In general, the switching frequency (on-time) and operating

point (% ripple or LIR) determine the inductor value as

shown in the following equation:

)

MAX

(

LOAD

OUT

IN

ON

I

LIR

)

V

V

(

t

L

×

−

×

=

where LIR is the ratio of the peak to peak ripple current to

the average inductor current.

Find a low loss inductor having the lowest possible DC

resistance that fits in the allotted dimensions. Ferrite cores

are often the best choices because powdered iron is

inexpensive and can work well at 200kHz. The core must

be large enough not to saturate at the peak inductor

current (I

)

MAX

(

LOAD

)

MAX

(

LOAD

PEAK

I

2

LIR

I

I

×

+

=

The calculation above shall serve as a general reference.

To further improve the transient response, the output

inductance can be reduced even further. This needs to

be considered along with the selection of the output

capacitor.

Output Capacitor Selection

The capacitor value and ESR determine the amount of

output voltage ripple and load transient response. Thus,

the capacitor value must be greater than the largest value

calculated from below equations:

OUT

OUT

2

LOAD

SOAR

V

C

2

L

I

V

×

×

×

∆

=

)

f

C

8

1

ESR

(

I

LIR

V

SW

OUT

)

MAX

(

LOAD

P

P

×

×

+

×

×

=

−

where V

voltage and overshoot voltage in the load transient, V

the output ripple voltage.

MOSFET Selection

The majority of power loss in the step-down power

converter is the loss in the power MOSFETs. For low

voltage high current applications, the duty cycle of the

upper MOSFET is small. Therefore, the switching loss of

the upper MOSFET is of concern. Power MOSFETs with

lower total gate charge are preferred in such kind of

application. However, the small duty cycle means the lower

MOSFET is on for most of the switching cycle. Therefore,

the conduction loss tends to dominate the total power loss

of the converter.

To improve the overall efficiency, MOSFETs with low R

DS(ON)

are preferred in the circuit design. In some cases, more

than one MOSFET are connected in parallel to further

decrease the on-state resistance. However, this depends

on the MOSFET driver capability and the budget.

Layout Considerations

Layout is very important in high frequency switching

converter designs, the PCB could radiate excessive noise

and contribute to the converter instability with improper

layout. Certain points must be considered before starting

a layout.

1

Place the filter capacitor close to the IC.

2

Keep current limit setting network as close as possible

to the IC. Routing of the network should avoid coupling

to high voltage switching node.

3

Connections from the drivers to the respective gate of

the both MOSFETs should be as short as possible to

reduce stray inductance.

4

All sensitive analog traces and components such as

VFB, GND, EN, POK, and RF should be placed away

from high voltage switching nodes such as PHASE,

LGATE, UGATE, or BOOT nodes to avoid coupling.

Use internal layer(s) as ground plane(s) and shield

the feedback trace from power traces and components.

5

Place the ground terminals of VIN capacitor(s), VOUT

capacitor(s), and source of lower MOSFETs as close

as possible. The PCB trace defined as PHASE node,

which connects to source of upper MOSFET, drain of

lower MOSFET and high voltage side of the inductor,

should be as short and wide as possible.