Datasheet

TPS54262-Q1
www.ti.com SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010
9) Resistor to Select Switching Frequency (R8)
Please refer to the section Selecting Switching Frequency, Figure 23 and Equation 7.
10) Resistors to Select Output Voltage (R4, R5)
To minimize the effect of leakage current on the VSENSE terminal, the current flowing through the feedback
network should be greater than 5 mA to maintain output accuracy. Higher resistor values help improve the
converter efficiency at low output currents, but may introduce noise immunity problems. Please refer to
Equation 1. It is recommended to fix R4 to a standard value (say 187 kΩ) and calculate R5.
11) Resistors to Set Undervoltage, Overvoltage, and Reset Thresholds (R1, R2, R3)
Overvoltage resistor selection
Using Equation 12, the value of R3 can be determined to set the overvoltage threshold at up to 106% to 110% of
V
Reg
. The sum of R1, R2, and R3 resistor network to ground should be is approximately 100 kΩ .
Reset threshold resistor selection
Using Equation 11 the value of R2 + R3 can be calculated, and knowing R3 from the OV_TH setting, R2 can be
determined. Suggested value of reset threshold is 92% of V
Reg
.
Undervoltage threshold for low-power mode and load transient operation
This threshold is set above the reset threshold to ensure the regulator operates within the specified tolerances
during output load transient of low load to high load and during discontinuous conduction mode. The typical
voltage threshold can be determined using Equation 10. Suggested value of undervoltage threshold is 95% of
V
Reg
.
Low-Power Mode (LPM) Threshold
An approximation of the output load current at which the converter is operating in discontinuous mode can be
obtained from Equation 4 with ± 30% hysteresis. The values used in Equation 6 for minimum and maximum input
voltage will affect the duty cycle and the overall discontinuous mode load current. These are the nominal values,
and other factors are not taken into consideration like external component variations with temperature and aging.
12) Pullup Resistor for Enable (R12)
An external pull resistor of 30.1 kΩ is recommended to enable the device for operation.
13) Type 3 Compensation Components (R5, R6, R9, C5, C7, C8)
First, make the 'ZEROs' close to double pole frequency, using Equation 15, Equation 16, and Equation 14.
fz1 = (50% to 70%) f
LC
fz2 = f
LC
Second, make the 'POLEs' above the crossover frequency, using Equation 21 and Equation 22.
fp1 = f
ESR
fp2 = ½f
sw
Resistors
From Equation 1, knowing V
Reg
and R4 (fix to a standard value), R5 can be calculated as shown in Equation 45:
(45)
Using Equation 14 and Equation 18, R6 can be calculated as shown in Equation 46:
(46)
R9 can be calculated as shown in Equation 47:
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