Datasheet

TPS54262-Q1
SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010 www.ti.com
Step 4. Calculate the Output Capacitor and ESR (C4)
Calculate capacitance
To calculate the capacitance of the output capacitor, minimum load current must be first determined. Typically, in
standby mode the load current is 100 µA, however this really depends on the application. With this value of
minimum load current and using Equation 35, Equation 36, and Equation 37, C4 is calculated to be, C4 > 34 µF .
To allow wider operating conditions and improved performance in low-power mode it is recommended to use a
100 µF capacitor. Higher value of output capacitor allows improved transient response during load stepping.
Calculate ESR
Using Equation 38, ESR is calculated to be, R
ESR
< 555 mΩ.
Capacitors with lowest ESR values should be selected. To meet both the requirements, capacitance and low
ESR, several low ESR capacitors may be connected in parallel. In this example, we will select a capacitor with
ESR value as 30 mΩ.
Filter capacitor (C12) of value 0.1µF can be added to filter out the noise in the output line.
Step 5. Calculate the Feedback Resistors (R4, R5)
To keep the quiescent current low and avoid instability problems, it is recommended to select R4 and R5 such
that, R4 + R5 ~ 250 kΩ.
Using Equation 1 and using a fixed standard value of R4 = 187 kΩ, R5 is calculated to be, R5 = 35.7 kΩ .
Step 6. Calculate Type 3 Compensation Components
Resistances (R6, R9)
Using Equation 19, for VIN
Typ
= 14 V, V
Ramp
is calculated to be, V
Ramp
= 1.4 V.
Using Equation 15, f
LC
is calculated to be, f
LC
= 3.33 kHz.
Using V
Ramp
, f
LC
from above, assuming f
c
as 1/10th of f
sw
and Equation 46, R6 is calculated to be, R6 = 280.65
kΩ.
Using Equation 47, R9 is calculated to be, R9 = 2.53 kΩ.
Capacitors (C5, C8, C7)
Using Equation 48, C5 is calculated to be, C5 = 340.45 pF.
Using Equation 16, f
ESR
is calculated to be, f
ESR
= 53.06 kHz.
Using Equation 50, C8 is calculated to be, C8 = 11.04 pF.
Using Equation 49, C7 is calculated to be, C7 = 250.07 pF.
Step 7. Calculate Soft-Start Capacitor (C6)
The recommended value of soft-start capacitor is 100nF (typical).
Step 8. Calculate Bootstrap Capacitor (C3)
The recommended value of bootstrap capacitor is 0.1 µF (typical).
Step 9. Calculate Power-On Reset Delay Capacitor (C2)
To achieve 2.2-ms delay, the reset delay capacitor can be calculated using Equation 9 to be C2 = 2.2 nF.
Step 10. Calculate Input Capacitor (C1, C11)
Typical values for C11 are 0.1 µF and 0.01 µF.
Input capacitor (C1) should be rated more than the maximum input voltage (VIN
Max
). The input capacitor should
be big enough to maintain supply in case of transients in the input line. Using Equation 34, C1 is calculated to
be, C1 = 1.2 µF. For improved transient response, a higher value of C1 such as 220 µF is recommended.
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