Datasheet

1
æ ö
-
ç ÷
è ø
OUT
ref
V
R6 = x R7
V
TPS5401
SLVSAB0 –DECEMBER 2010 www.ti.com
Switching Frequency
The first step is to decide on a switching frequency for the regulator. Typically, the user may want to choose the
highest switching frequency possible, because this produces 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. Alternatively, the user may choose a lower switching frequency to improve
efficiency. At lower switching frequencies, switching losses are minimized. The switching frequency that can be
selected is limited by the minimum on-time of the internal power switch, the input voltage, the output voltage, and
the frequency shift limitation.
Equation 10 and Equation 11 must be used to find the maximum switching frequency for the regulator; choose
the lower value of the two equations. Switching frequencies higher than these values result in pulse skipping or
the lack of overcurrent protection during a short circuit.
The typical minimum on-time, t
on(min)
, is 130 ns for the TPS5401. For this example, the output voltage is 5 V and
the maximum input voltage is 35 V, which allows for a maximum switch frequency up to 1213 kHz when including
the inductor resistance, on-resistance, and diode voltage in Equation 10. To ensure overcurrent runaway is not a
concern during short circuits in your design, use Equation 11 or the solid curve in Figure 38 to determine the
maximum switching frequency. With a maximum input voltage of 35 V, assuming a diode voltage of 0.5 V,
inductor resistance of 130 mΩ, switch resistance of 400 mΩ, a current-limit value of 0.94 A, and a short-circuit
output voltage of 0.1 V, the maximum switching frequency is approximately 1265 kHz. Choosing high frequency
can reduce external component size but results in higher switching loss. To achieve a balanced design, a
switching frequency of 700 kHz is used. To determine the timing resistance for a given switching frequency, use
Equation 9 to get a nearest standard resistance of 165 kΩ. The switching frequency is set to 698 kHz by the
resistor R3 shown in Figure 40.
Output Voltage Setpoint
The output voltage of the TPS5401 is externally adjustable using a resistor divider network. In the application
circuit of Figure 40, this divider network is comprised of R6 and R7. The relationship of the output voltage to the
resistor divider is given by Equation 12:
(12)
Choosing R7 = 10 kΩ, R6 is calculated to be 52.3 kΩ for an output voltage of 5 V.
Due to current leakage of the VSENSE pin, the current flowing through the feedback network should be greater
than 1 mA in order to maintain the output voltage accuracy. This requirement makes the maximum value of R7
equal to 800 kΩ. Choosing higher resistor values decreases quiescent current and improve efficiency at low
output currents but may introduce noise immunity problems.
Input Capacitor
The TPS5401 requires a high-quality ceramic, type X5R or X7R, input decoupling capacitor with at least 3 mF of
effective capacitance, and in some applications a bulk capacitance. The effective capacitance includes any dc
bias effects. The voltage rating of the input capacitor must be greater than the maximum input voltage. The
capacitor must also have a ripple-current rating greater than the maximum input-current ripple of the TPS5401.
The input ripple current can be calculated using Equation 13.
The value of a ceramic capacitor varies significantly over temperature and the amount of dc bias applied to the
capacitor. The capacitance variations due to temperature can be minimized by selecting a dielectric material that
is stable over temperature. X5R and X7R ceramic dielectrics are usually selected for power regulator capacitors
because they have a high capacitance-to-volume ratio and are fairly stable over temperature. The output
capacitor must also be selected with the dc bias taken into account. The capacitance value of a capacitor
decreases as the dc bias across a capacitor increases.
For this example design, a ceramic capacitor with at least a 60-V voltage rating is required to support the
maximum input voltage. Common standard ceramic capacitor voltage ratings include 4 V, 6.3 V, 10 V, 16 V, 25
V, 50 V, and 100 V, so a 100-V capacitor should be selected. For this example, two 2.2-mF, 100-V capacitors in
parallel have been selected. Table 4 shows a selection of high-voltage capacitors. The input capacitance value
determines the input ripple voltage of the regulator. The input voltage ripple can be calculated using Equation 14.
24 Submit Documentation Feedback Copyright © 2010, Texas Instruments Incorporated
Product Folder Link(s): TPS5401