Datasheet

L =
(V
IN
- V
OUT
)V
OUT
V
IN
x I
RIPPLE
x f
SW
LMR14203
www.ti.com 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
state), the buck regulator operates in two cycles. The power switch is connected between V
IN
and SW. In the first
cycle of operation the transistor is closed and the diode is reverse biased. Energy is collected in the inductor and
the load current is supplied by C
OUT
and the rising current through the inductor. During the second cycle the
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:
D=V
OUT
/V
IN
and D’ = (1-D)
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:
V
OUT
=0.765V(1+(R1/R2)) (2)
Typically R2 will be given as 100Ω-10 kΩ for a starting value. To solve for R1 given R2 and V
OUT
use
R1=R2((V
OUT
/0.765V)-1).
Input Capacitor
A low ESR ceramic capacitor (C
IN
) is needed between the V
IN
pin and GND pin. This capacitor prevents large
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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