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LMR14203

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SNVS732C – OCTOBER 2011 – REVISED APRIL 2013

APPLICATION INFORMATION

Protection

The LMR14203 has dedicated protection circuitry running during normal operation to protect the IC. The thermal

shutdown circuitry turns off the power device when the die temperature reaches excessive levels. The UVLO

comparator protects the power device during supply power startup and shutdown to prevent operation at

voltages less than the minimum input voltage. A gate drive (CB) under-voltage lockout is included to ensure that

there is enough gate drive voltage to drive the MOSFET before the device tries to start switching. The

LMR14203 also features a shutdown mode decreasing the supply current to approximately 16 µA.

Continuous Conduction Mode

The LMR14203 contains a current-mode, PWM buck regulator. A buck regulator steps the input voltage down to

a lower output voltage. In continuous conduction mode (when the inductor current never reaches zero at steady

cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and

transistor is open and the diode is forward biased due to the fact that the inductor current cannot instantaneously

change direction. The energy stored in the inductor is transferred to the load and output capacitor. The ratio of

these two cycles determines the output voltage. The output voltage is defined approximately as:

where

•

D is the duty cycle of the switch.

(1)

D and D' will be required for design calculations.

Design Procedure

This section presents guidelines for selecting external components.

Setting the Output Voltage

The output voltage is set using the feedback pin and a resistor divider connected to the output as shown on the

front page schematic. The feedback pin voltage is 0.765V, so the ratio of the feedback resistors sets the output

voltage according to the following equation:

(2)

Typically R2 will be given as 100

Input Capacitor

voltage transients from appearing at the input. Use a 2.2 µF-10 µF value with X5R or X7R dielectric. Depending

on construction, a ceramic capacitor’s value can decrease up to 50% of its nominal value when rated voltage is

applied. Consult with the capacitor manufacturer's data sheet for information on capacitor derating over voltage

and temperature.

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.

(3)

A higher value of ripple current reduces inductance, but increases the conductance loss, core loss, and current

stress for the inductor and switch devices. It also requires a bigger output capacitor for the same output voltage

ripple requirement. A reasonable value is setting the ripple current to be 30% of the DC output current. Since the

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

inductance. The DC resistance of the inductor is a key parameter for the efficiency. Lower DC resistance is

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