LMR14030-Q1 Datasheet, PDF (19/32 Pages) Texas Instruments – SIMPLE SWITCHER 40 V, 3.5A Step-Down Converter

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LMR14030-Q1
SNVSAG3 – NOVEMBER 2015
The two components in the voltage ripple are not in phase, so the actual peak-to-peak ripple is smaller than the
sum of two peaks.
Output capacitance is usually limited by transient performance specifications if the system requires tight voltage
regulation with presence of large current steps and fast slew rate. When a fast large load increase happens,
output capacitors provide the required charge before the inductor current can slew up to the appropriate level.
The regulator’s control loop usually needs three or more clock cycles to respond to the output voltage droop. The
output capacitance must be large enough to supply the current difference for three clock cycles to maintain the
output voltage within the specified range. Equation 13
Equation 13 shows the minimum output capacitance needed for specified output undershoot. When a sudden
large load decrease happens, the output capacitors absorb energy stored in the inductor. The catch diode can’t
sink current so the energy stored in the inductor results in an output voltage overshoot. Equation 14 calculates
the minimum capacitance required to keep the voltage overshoot within a specified range.
C_OUT
!
3 u (I_OH I_OL )
¦_SW u 9_US
(13)
C_OUT
!
(V_OUT
I_{O2 H} I_{O2 L}
V_OS )²
V_O2UT ^{u L}
(14)
where
• K_IND = Ripple ratio of the inductor ripple current (Δi_L / I_OUT)
• I_OL = Low level output current during load transient
• I_OH = High level output current during load transient
• V_US = Target output voltage undershoot
• V_OS = Target output voltage overshoot
For this design example, the target output ripple is 50 mV. Presuppose ΔV_{OUT_ESR} = ΔV_{OUT_C} = 50 mV, and
chose K_IND = 0.4. Equation 11 yields ESR no larger than 35.7 mΩ and Equation 12 yields C_OUT no smaller than 7
μF. For the target over/undershoot range of this design, V_US = V_OS = 5% × V_OUT = 250 mV. The C_OUT can be
calculated to be no smaller than 75.6 μF and 30.8 μF by Equation 13 and Equation 14 respectively. In summary,
the most stringent criteria for the output capacitor is 75.6 μF. Two 47 μF, 16 V, X7R ceramic capacitors with 5
mΩ ESR are used in parallel.
8.2.2.5 Schottky Diode Selection
The breakdown voltage rating of the diode is preferred to be 25% higher than the maximum input voltage. The
current rating for the diode should be equal to the maximum output current for best reliability in most
applications. In cases where the input voltage is much greater than the output voltage the average diode current
is lower. In this case it is possible to use a diode with a lower average current rating, approximately (1-D) × I_OUT
however the peak current rating should be higher than the maximum load current. A 4 A to 5 A rated diode is a
good starting point.
8.2.2.6 Input Capacitor Selection
The LMR14030-Q1 device requires high frequency input decoupling capacitor(s) and a bulk input capacitor,
depending on the application. The typical recommended value for the high frequency decoupling capacitor is 4.7
μF to 10 μF. A high-quality ceramic capacitor type X5R or X7R with sufficiency voltage rating is recommended.
To compensate the derating of ceramic capacitors, a voltage rating of twice the maximum input voltage is
recommended. Additionally, some bulk capacitance can be required, especially if the LMR14030-Q1 circuit is not
located within approximately 5 cm from the input voltage source. This capacitor is used to provide damping to the
voltage spike due to the lead inductance of the cable or the trace. For this design, two 2.2 μF, X7R ceramic
capacitors rated for 100 V are used. A 0.1 μF for high-frequency filtering and place it as close as possible to the
device pins.
8.2.2.7 Bootstrap Capacitor Selection
Every LMR14030-Q1 design requires a bootstrap capacitor (C_BOOT). The recommended capacitor is 0.1 μF and
rated 16 V or higher. The bootstrap capacitor is located between the SW pin and the BOOT pin. The bootstrap
capacitor must be a high-quality ceramic type with an X7R or X5R grade dielectric for temperature stability.
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