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TPS64202 Datasheet(PDF) 10 Page - Texas Instruments |
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TPS64202 Datasheet(HTML) 10 Page - Texas Instruments |
10 / 24 page TPS64200,TPS64201 TPS64202,TPS64203 SLVS485 − AUGUST 2003 www.ti.com 10 Figure 18 TPS64200 STARTUP TIMING EN VO II I(Inductor) VI = 3.8 V, VO = 3.3 V, RL = 1.66 Ω, TA = 25°C 100 ms/Div DETAILED DESCRIPTION Operation The TPS6420x is a nonsynchronous step-down controller which is operating with a minimum on-time/minimum off-time control. An external PMOS is turned on until the output voltage reaches its nominal value or the current limit is exceeded. If the current limit is exceeded, the PMOS is switched off and stays off for the minimum off-time. After that the PMOS is switched on again. When the nominal output voltage is reached, the PMOS is switched off and stays off until the output voltage dropped below its nominal value. Operating Modes When delivering low or medium output current, the TPS6420x operate in discontinuous mode. With every switching cycle, the current in the inductor starts at zero, rises to a maximum value and ramps down to zero again. As soon as the current in the inductor drops to zero, ringing occurs at the resonant frequency of the inductor and stray capacitance, due to residual energy in the inductor when the diode turns off. Ringing in discontinuous mode is normal and does not have any influence on efficiency. The ringing does not contain much energy and can easily be damped by an RC snubber. See the application section for further details. With high output current, the TPS6420x operate in continuous current mode. In this mode, the inductor current does not drop to zero within one switching cycle. The output voltage in continuous mode is directly dependant on the duty cycle of the switch. Variable Minimum On-Time (TPS64201 to TPS64202 Only) The minimum on-time of the device is 1.6 µs. At light loads, this would cause a low switching frequency in the audible range because the energy transferred to the output during the on-time would cause a higher rise in the output voltage than needed and therefore lead to a long off−time until the output voltage dropped again. To avoid a switching frequency in the audible range the TPS64201 and TPS64202 can internally reduce the minimum on time in three steps from 1.6 µs to 800 ns, 400 ns and 200 ns. The on-time is reduced by one step if the switching frequency dropped to a lower value than 50 kHz. This keeps the frequency above the audio frequency over a wide load range and also keeps the output voltage ripple low. |
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