Datasheet

TPS54262-Q1
www.ti.com SLVS996C –SEPTEMBER 2009–REVISED JUNE 2010
Overvoltage Supervisor
The overvoltage monitoring of the regulated output voltage, V
Reg
can be achieved by connecting an external
resistor string to the OV_TH pin (pin 12). The resistor combination of R1, R2, and R3 is used to program the
threshold for detection of overvoltage. The bias voltage of R3 sets the overvoltage threshold and the accuracy of
regulated output voltage in hysteretic mode during transient events.
(12)
Recommended range for VReg_OV is 106% to 110% of V
Reg
.
Noise Filter on RST_TH and OV_TH Terminals
External capacitors may be required to filter the noise added to RST_TH and OV_TH terminals. The noise is
more pronounced with fast falling edges on the PH pin. Therefore, selecting a smaller Rslew resistor (R7 in
Figure 4) for a higher slew rate will require more external capacitance to filter the noise.
The RC time constant depends on external components (R2, R3, C9 and C10 in Figure 4) connected to RST_TH
and OV_TH pins. For proper noise filtering, improved loop transient response and better short circuit protection,
Equation 13 must be satisfied.
(R2 + R3) × (C9 + C10) < 2 µs (13) (13)
To meet this requirement, it is recommended to use lower values of external capacitors and resistors. The value
of the time constant is also affected by the PCB capacitance and the application setup. Therefore, in some cases
the external capacitors (C9, C10) on RST_TH and OV_TH terminals may not be required. Users can place a
footprint on the application PCB and only populate it if necessary. Also, the external resistors (R1, R2, R3)
should be sized appropriately to minimize any significant effect of board leakage.
For most cases, it is recommended to keep the external capacitors (either from board capacitance or by
connecting external capacitors) between 10 pF to 100 pF; therefore, to meet time constant requirement in
Equation 13, the total external resistance (R1 + R2 + R3) should be less than 200 kΩ.
Boost Capacitor
An external boot strap capacitor (C3 in Figure 4) is connected to pin 20 to provide the gate drive voltage for the
internal NMOS switching FET. X7R or X5R grade dielectrics are recommended due to their stable values over
temperature. The capacitor value may need to be adjusted higher for high V
Reg
and/or low frequencies
applications (e.g., 100 nF for 500 kHz/5 V and 220 nF for 500 kHz/8 V).
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