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HT77S11 Datasheet(PDF) 7 Page - Holtek Semiconductor Inc |
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HT77S11 Datasheet(HTML) 7 Page - Holtek Semiconductor Inc |
7 / 16 page HT77S10/HT77S11 Rev. 1.00 7 February 3, 2010 Ripple Voltage vs Output Current 0 20 40 60 80 100 120 140 0 50 100 150 200 250 300 350 400 450 500 550 Output Current (mA) HT77S10 VOUT=3.3V L=22uH CIN=100uF COUT=100uF VIN=1.2V VIN=2.4V Functional Description These devices use minimum off-time, current limited pulse-frequency modulation, PFM, techniques for out- put voltage regulation. The PFM control circuitry acts to control the energy stored in an external inductor to regu- late the output voltage to the correct level. Using PFM for control offers the advantages of very low quiescent currents. The device output voltage is continuously monitored us- ing an internal comparator. The output status of the comparator is then used to determine what action the control logic should take to correct any differences be- tween the actual output voltage and the required volt- age. The variable switching frequency depends upon the input voltage and the load demands but can have a frequency of up to 500kHz. The maximum current of the internal NMOS power switching transistor is fixed at 1A for the HT77S10 and 0.55A for the HT77S11. The switching frequency is controlled by two internal single shot circuits that generate pulses of 0.9 ms typical minimum off-time and 4 ms typical maximum on-time. If the circuitry detects that the output voltage is lower than the required voltage, the control circuitry will ensure that more inductor energy is transferred to the load. Refer to the block diagram to see how this regulation is imple- mented using the following two ON and OFF steps: · ON Step During this step, the NMOS transistor will be switched on which effectively connects the LX pin to ground, and allows energy to build up in the inductor as sup- plied by VIN. Inductor energy build up is allowed to continue until the inductor current reaches a maxi- mum of 1A for the HT77S10 and 0.55A for the HT77S11 or if the on time exceeds 4 ms. · OFF Step The energy stored in the inductor during the ON Step is now transferred to the load via the synchronous switch for a minimum time of 0.9 ms. When the output voltage is less than the required regulated voltage the synchronous switch will switch off which will reduce the inductor current to zero. It is during this cycle that the PMOS power transistor is switched on which shunts its body diode, resulting in enhanced conver- sion efficiency. As the design of these devices uses an internal synchro- nous rectifier, there is no requirement to provide the usual external Schottky diode. This gives users the ad- vantage of smaller circuit board areas and reduced costs. Low Voltage Start-up These devices can provide a very low start up voltage down to 0.7V. When power is first applied, the synchro- nous switch will be initially off but energy will be trans- ferred to the load through its intrinsic body diode. Voltage Reference The internal voltage reference circuit supplies a nominal voltage of 1.3V on the REF pin. Up to 150 mA can be sourced on this pin for use by external circuits. The ref- erence voltage provided here offers excellent load regu- lation characteristics, however a bypass capacitor of 0.1 mF should be connected for proper operation. |
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