NCV51411 datasheet(10/16 Pages) ONSEMI

Part #	NCV51411

Description	1.5 A, 260 kHz, Low Voltage Buck Regulator with Synchronization Capability

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NCV51411

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10

WBASE +

VO2

VIN

IS

60

where:

IS = DC switching current.

When the power switch turns on, the saturation voltage

and conduction current contribute to the power loss of a

non−ideal switch. The power loss can be quantified as

WSAT +

VO

VIN

IS

VSAT

where:

VSAT = saturation voltage of the power switch which is

shown in Figure 5.

The switching loss occurs when the switch experiences

both high current and voltage during each switch transition.

This regulator has a 30 ns turn−off time and associated

power loss is equal to

WS +

IS

VIN

2

30 ns

fS

The turn−on time is much shorter and thus turn−on loss is

not considered here.

The total power dissipated by the IC is sum of all the above

WIC + WQ ) WDRV ) WBASE ) WSAT ) WS

The IC junction temperature can be calculated from the

ambient temperature, IC power dissipation and thermal

resistance of the package. The equation is shown as follows,

TJ + WIC

RqJA ) TA

Minimum Load Requirement

As pointed out in the previous section, a minimum load is

required for this regulator due to the pre−driver current

feeding the output. Placing a resistor equal to VO divided by

12 mA should prevent any voltage overshoot at light load

conditions. Alternatively, the feedback resistors can be

valued properly to consume 12 mA current.

COMPONENT SELECTION

Input Capacitor

In a buck converter, the input capacitor witnesses pulsed

current with an amplitude equal to the load current. This

pulsed current and the ESR of the input capacitors determine

the VIN ripple voltage, which is shown in Figure 11. For VIN

ripple, low ESR is a critical requirement for the input

capacitor selection. The pulsed input current possesses a

significant AC component, which is absorbed by the input

capacitors. The RMS current of the input capacitor can be

calculated using:

IRMS + IO D(1 * D)

where:

D = switching duty cycle which is equal to VO/VIN.

IO = load current.

Figure 11. Input Voltage Ripple in a Buck Converter

To calculate the RMS current, multiply the load current

with the constant given by Figure 12 at each duty cycle. It

is a common practice to select the input capacitor with an

RMS current rating more than half the maximum load

current. If multiple capacitors are paralleled, the RMS

current for each capacitor should be the total current divided

by the number of capacitors.

Figure 12. Input Capacitor RMS Current can be

Calculated by Multiplying Y Value with Maximum Load

Current at any Duty Cycle

0

0.2

0.4

1.0

DUTY CYCLE

0

0.1

0.3

0.4

0.5

0.6

0.2

0.6

0.8

Selecting the capacitor type is determined by each

design’s constraint and emphasis. The aluminum

electrolytic capacitors are widely available at lowest cost.

Their ESR and ESL (equivalent series inductor) are

relatively high. Multiple capacitors are usually paralleled to

achieve lower ESR. In addition, electrolytic capacitors

usually need to be paralleled with a ceramic capacitor for

filtering high frequency noises. The OS−CON are solid

aluminum electrolytic capacitors, and therefore has a much

lower ESR. Recently, the price of the OS−CON capacitors

has dropped significantly so that it is now feasible to use

them for some low cost designs. Electrolytic capacitors are

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