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TPS54550 Fiches technique(PDF) 8 Page - Texas Instruments |
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TPS54550 Fiches technique(HTML) 8 Page - Texas Instruments |
8 / 27 page www.ti.com DETAILED DESCRIPTION Undervoltage Lockout (UVLO) Slow Start Enable (SSENA) and Internal Slow T SS_INTERNAL(ms) 1.15k ƒ s(kHz) (3) 5 µA Disabled Enabled CSS 320 k Ω 125 k Ω R1 R2 Input Voltage Supply 1 k Ω Extending Slow Start Time R1 VIN(start) 1 kp 1.24 V ) 1kp (1) VIN(stop) (R1 ) 1 kp) 1.02 V 1 kp (2) TPS54550 SLVS623A – MARCH 2006 – REVISED APRIL 2006 Start The undervoltage lockout (UVLO) system has an internal voltage divider from VIN to AGND. The Once the SSENA pin voltage exceeds 0.5 V, the defaults for the start/stop values are labeled VIN and TPS54550 starts operation. The TPS54550 has an given in Table 1. The internal UVLO threshold can be internal digital slow start that ramps the reference overridden by placing an external resistor divider from voltage to its final value in 1150 switching cycles. The VIN to ground. The internal divider values are internal slow start time (10% - 90%) is approximated approximately 320 k Ω for the high side resistor and by the following expression: 125 k Ω for the low side resistor. The divider ratio (and therefore the default start/stop values) is quite accurate, but the absolute values of the internal resistors may vary as much as 15%. If high accuracy Once the TPS54550 device is in normal regulation, is required for an externally adjusted UVLO threshold, the SSENA pin is high. If the SSENA pin is pulled select lower value external resistors to set the UVLO below the stop threshold of 0.5 V, switching stops threshold. Using a 1-k Ω resistor for the low side and the internal slow start resets. If an application resistor R2 (see Figure 1) is recommended. Under no requires the TPS54550 to be disabled, use open circumstances should the UVLO pin be connected drain or open collector output logic to interface to the directly to VIN. SSENA pin (see Figure 2). The SSENA pin has an internal pullup current source. Do not use external Table 1. Start/Stop Voltage Threshold pullup resistors. START VOLTAGE STOP VOLTAGE THRESHOLD THRESHOLD VIN (Default) 4.49 3.69 UVLO 1.24 1.02 Figure 2. Interfacing to the SSENA Pin Figure 1. Circuit Using External UVLO Function The equations for selecting the UVLO resistors are: In applications that use large values of output capacitance there may be a need to extend the slow start time to prevent the startup current from tripping the current limit. The current limit circuit is designed to disable the high side MOSFET and reset the internal voltage reference for a short amount of time For applications which require an undervoltage lock when the high side MOSFET current exceeds the out (UVLO) threshold greater than 4.49 V, external current limit threshold. If the output capacitance and resistors may be implemented, see Figure 1, to adjust load current cause the startup current to exceed the the start voltage threshold. For example, an current limit threshold, the power supply output will application needing an UVLO start voltage of not reach the desired output voltage. To extend the approximately 7.8 V using the equation (1), R1 is slow start time and to reduce the startup current, an calculated to the nearest standard resistor value of external capacitor can be added to the SSENA pin. 5.36 k Ω. Using Equation 2, the input voltage stop The slow start capacitance is calculated using the threshold is calculated as 6.48 V. following equation: CSS(µF) = 5.55x10 -3 T ss(ms) 8 Submit Documentation Feedback |
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