2





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I

I

I

2

2

1

12



L-RMS

o

ripple

I

I

I

max

max

(

)

K



K

K

out

in

out

ripple

in

o

sw

V

V

V

I

V

L

f

max

min

max



K

K

K

in

out

out

o

o

IND

in

sw

V

V

V

L

I

K

V

f

LMR14003, LMR14006

SNVSA10 – NOVEMBER 2013

www.ti.com

APPLICATION INFORMATION

Design Guide – Step By Step Design Procedure

This example details the design of a high frequency switching regulator using ceramic output capacitors. A few

parameters must be known in order to start the design process. These parameters are typically determined at the

system level:

Input Voltage, VIN

9V to 16V, Typical 12V

Output Voltage, VOUT

5.0V ± 3%

Maximum Output Current IO_max

0.6A

Minimum Output Current IO_min

0.03A

Transient Response 0.03A to 0.6A

5%

Output Voltage Ripple

1%

Switching Frequency Fsw

2.1MHz

Target during Load Transient

Over Voltage Peak Value

106% of Output Voltage

Under Voltage Value

91% of Output Voltage

Selecting the Switching Frequency

The first step is to decide on a switching frequency for the regulator. Typically, the user will want to choose the

highest switching frequency possible since this will produce the smallest solution size. The high switching

frequency allows for lower valued inductors and smaller output capacitors compared to a power supply that

switches at a lower frequency. The switching frequency that can be selected is limited by the minimum on-time of

the internal power switch, the input voltage and the output voltage and the frequency shift limitation. For this

example, the output voltage is 5V and the maximum input voltage is 16V, a switching frequency of 2100kHz is

used.

Output Inductor Selection

The most critical parameters for the inductor are the inductance, peak current and the DC resistance. The

inductance is related to the peak-to-peak inductor ripple current, the input and the output voltages. Since the

ripple current increases with the input voltage, the maximum input voltage is always used to determine the

represents the amount of inductor ripple current relative to the maximum output current. A reasonable value is

setting the ripple current to be 30% of the DC output current. For this design example, the minimum inductor

value is calculated to be 9.1 µH, and a nearest standard value was chosen: 10 µH. For the output filter inductor,

it is important that the RMS current and saturation current ratings not be exceeded. The RMS and peak inductor

current can be found from Equation 3 and Equation 4. The inductor ripple current is 0.16A, and the RMS current

is 0.602A. As the equation set demonstrates, lower ripple currents will reduce the output voltage ripple of the

regulator but will require a larger value of inductance. A good starting point for most applications is a 10

μH with

2A current rating. Using a rating near 2A will enable the LMR14003/6 to current limit without saturating the

inductor. This is preferable to the LMR14003/6 going into thermal shutdown mode and the possibility of

damaging the inductor if the output is shorted to ground or other long-term overload.

(1)

(2)

(3)

(4)

10

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