TPS5401_15 Datasheet, PDF (33/45 Pages) Texas Instruments – TPS5401 0.5-A, 42-V Input, Step-Down SWIFT™ Converter

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TPS5401
SLVSAB0A – DECEMBER 2010 – REVISED NOVEMBER 2014
The first step is to choose the closed-loop crossover frequency f_CO. In general, the closed-loop crossover
frequency could be less than 1/10 of the minimum operating frequency. For the TPS5401, the maximum closed-
loop crossover frequency must not be not greater than 40 kHz. For this example, we choose 15-kHz crossover
frequency. Next, by definition, the magnitude of the loop gain at the crossover frequency is 0 dB. By placing the
compensation zero at the power stage pole, and the mid-compensation pole at the power stage zero, the R_C, C_C
and C_P can be approximately calculated by Equation 33 through Equation 35.
R_C
=
2p ´ f_CO ´ C_{OUT ×} V_OUT
gm_PS
V_ref ´ gm_EA
(33)
1
C_C = _{2p ´ fPmod}
(34)
1
C_P = _{2p ´ fZmod}
where
• gm_PS is the power-stage transconductance. It is 1.9 A/V
• gm_EA is the error-amplifier transconductance. It is 97 μA/V
(35)
For this design, the calculated values are as follows:
• R3 = 37.4 kΩ
• C6 = 2200 pF
• C7 = 22 pF
9.2.2.11 Discontinuous Mode and Eco-mode Control-Scheme Boundary
With an input voltage of 34 V, the power supply enters discontinuous mode when the output current is less than
50 mA. The power supply enters Eco-mode control scheme when the output current is lower than 30 mA.
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