TPS54232_15 Datasheet, PDF (19/31 Pages) Texas Instruments – TPS54232 2-A, 28-V, 1-MHz, Step-Down DC-DC Converter With Eco-Mode™

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TPS54232
SLVS876D – NOVEMBER 2008 – REVISED NOVEMBER 2014
V_O = Output voltage
C_O = Output capacitance
F_CO = Desired crossover frequency
R_OA = 8.696 MΩ
GM_COMP = 10 A/V
V_ggm = 800
V_REF = 0.8 V
With R_Z known, C_Z and C_P can be calculated using Equation 27 and Equation 28.
C
Z
=
2´p
1
´
F
Z
1
´
R
z
(27)
C
P
=
2´p
1
´
F
P1
´
R
z
(28)
For this design, a 22-μF output capacitor issued. For ceramic capacitors, the actual output capacitance is less
than the rated value when the capacitors have a dc bias voltage applied. This is the case in a DC - DC converter.
For this design, a 10-V capacitor is chosen to minimize this effect. The ESR is approximately 0.005 Ω.
Using Equation 20 and Equation 21, the output stage gain and phase loss are equivalent as:
Gain = 1.613 dB
and
PL = -92.3 degrees
For 60 degrees of phase margin, Equation 22 requires 62.33 degrees of phase boost.
Equation 23, Equation 24, and Equation 25 are used to find the zero and pole frequencies of:
F_Z1 = 12.3 k Hz
And
F_P1 = 203 kHz
R_Z, C_Z, and C_P are calculated using Equation 26, Equation 27, and Equation 28.
2 ´ p ´ 50000 ´ 2.5 ´ 22 ´ 10^-6 ´ 8.696 ´ 10⁶ ´ 0.754
Rz =
= 17.7 kW
10 ´ 800 ´ 0.8
(29)
Cz =
1
= 730 pF
2 ´ p ´ 12300 ´ 17700
(30)
Cp =
1
= 44 pF
2 ´ p ´ 203000 ´ 17700
(31)
Using standard values for R3, C6, and C7 in the application schematic of Figure 10:
R3 = 17.4 kΩ
C6 = 680 pF
C7 = 47 pF
9.2.2.6 Bootstrap Capacitor
Every TPS54232 design requires a bootstrap capacitor, C4. The bootstrap capacitor must be 0.1 μF. The
bootstrap capacitor is located between the PH pins and BOOT pin. The bootstrap capacitor should be a high-
quality ceramic type with X7R or X5R grade dielectric for temperature stability.
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