ADP5003ACPZ-R7 datasheet(22/31 Pages) AD

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ADP5003

Data Sheet

Rev. A | Page 22 of 31

BUCK REGULATOR DESIGN EXAMPLE

This section provides an example of the step by step design

procedures and the external components required for the buck

regulator. Table 10 lists the design requirements for this

example.

Table 10. Example Design Requirements for the Buck Regulator

Parameter

Specification

Input Voltage

VPVIN1 = 12 V

Output Voltage

VPVOUT1 = 2.5 V

Output Current

ILOAD1 = 3 A

Output Ripple

ΔVOUT1_RIPPLE = 25 mV

Load Transient

±5% at 20% to 80% load transient

SETTING THE SWITCHING FREQUENCY FOR THE

BUCK REGULATOR

The first step is to determine the switching frequency for the

ADP5003 design. In general, higher switching frequencies

produce a smaller solution size due to the lower component

values required, whereas lower switching frequencies result in

higher conversion efficiency due to lower switching losses.

The switching frequency of the ADP5003 can be set from

0.3 MHz to 2.5 MHz by connecting a resistor from the RT pin

to ground. The selected resistor allows the user to make decisions

based on the trade-off between efficiency and solution size. (For

more information, see the Oscillator Frequency Control section.)

However, the highest supported switching frequency must be

assessed by checking the voltage conversion limitations enforced

by the minimum on time and the minimum off time (see the

Voltage Conversion Limitations section).

In this design example, a switching frequency of 600 kHz is

used to achieve an ideal combination of small solution size and

high conversion efficiency. To set the switching frequency to

600 kHz, use Equation 1 to calculate the resistor value, RRT. This

gives a standard resistor value of RT = 294 kΩ.

SETTING THE OUTPUT VOLTAGE FOR THE BUCK

REGULATOR

Select a value for the top resistor (RTOP1) and then calculate the

bottom feedback (RBOT1) resistor by using the following

equation:

RBOT1 = (RTOP1 × VPVOUT1)/((VREFOUT × ABUCK) − VPVOUT1) (16)

where:

VPVOUT1 is the buck output voltage.

VREFOUT is 2 V.

ABUCK is the buck regulator gain.

To set the output voltage to 2.5 V, RTOP1 is set to 100 kΩ, giving

an RBOT1 value of 100 kΩ.

SELECTING THE INDUCTOR FOR THE BUCK

REGULATOR

The peak-to-peak inductor ripple current, ΔIL, is set to 35% of

the maximum output current. Use Equation 8 to estimate the

value of the inductor:

L = ((VPVIN1 − VPVOUT1) × D)/(ΔIL × fSW)

where:

VPVIN1 = 12 V.

VPVOUT1 = 2.5 V.

D is the duty cycle (D = VPVOUT1/VPVIN1).

ΔIL = 35% × 3 A = 1.05 A.

fSW = 600 kHz.

The resulting value for L is 3.14 µH. The selected standard

inductor value is 3.3 µH; therefore, ΔIL is 1 A.

To calculate the peak inductor current (IPEAK), use Equation 9:

IPEAK = ILOAD1 + (ΔIL/2)

The calculated peak current for the inductor is 3.5 A.

SELECTING THE OUTPUT CAPACITOR FOR THE

BUCK REGULATOR

The output capacitor must meet the output voltage ripple, load

transient requirements and stability requirements. To meet the

output voltage ripple requirement, use Equation 7 to calculate

the capacitance:

)

(

—

ESR

RIPPLE

OUT_MIN

ΔI

≅

The calculated capacitance, COUT_MIN, is 8.7 µF.

To meet the ±5% overshoot and undershoot requirements, use

the following equations to calculate the capacitance:

(

)

OUT

PVOUT

PVIN

STEP

OUT_UV

ΔV

ΔI

–

(17)

(

)

– PVOUT

OUT

PVOUT

STEP

OUT_OV

ΔV

ΔI

(18)

where:

KUV and KOV are factors (typically set to 2).

ΔISTEP is the load step.

ΔVOUT_UV is the allowable undershoot on the output voltage.

ΔVOUT_OV is the allowable overshoot on the output voltage.

For estimation purposes, use KOV = KUV = 2; therefore,

COUT_OV = 33.4 µF and COUT_UV = 9 µF.

It is recommended to use two 22 µF ceramic capacitors.

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