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US1175CS Datasheet(PDF) 4 Page - UNISEM |
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US1175CS Datasheet(HTML) 4 Page - UNISEM |
4 / 5 page US1175 2-4 Rev. 1.2 11/29/99 APPLICATION INFORMATION APPLICATION INFORMATION Introduction The US1175 adjustable regulator is a 5 terminal device designed specificaly to provide extremely low dropout voltages comparable to the PNP type without the disad- vantage of the extra power dissipation due to the base current associated with PNP regulators.This is done by bringinging out the control pin of the regulator that pro- vides the base current to the power NPN and connect- ing it to a voltage that is grater than the voltage present at the Vin pin.This flexebility makes the US1175 ideal for applications where dual inputs are available such as a computer motherboard with an ATX style power sup- ply that provides 5V and 3.3V to the board.One such application is the new graphic chip sets that require any- where from 2.4V to 2.7V supply such as the Intel I740 chip set. The US1175 can easily be programmed with the addition of two external resistors to any voltages within the range of 1.25 to 5.5 V. Another major require- ment of these graphic chips is the need to switch the load current from zero to several amps in tens of nano- seconds at the processor pins ,which translates to an approximately 300 to 500 nS of current step at the regu- lator . In addition, the output voltage tolerances are also extremely tight and they include the transient response as part of the specification. The US1175 is specifically designed to meet the fast current transient needs as well as providing an accurate initial voltage , reducing the overall system cost with the need for fewer number of output capacitors.Another fea- ture of the device is its true remote sensing capability which allows accurate voltage setting at the load rather than at the device. Output Voltage Setting The US1175 can be programmed to any voltages in the range of 1.25V to 5.5V with the addition of R1 and R2 external resistors according to the following formula: Figure 2 - Typical application of the US1175 for programming the output voltage. BLOCK DIAGRAM BLOCK DIAGRAM V V R R I R Wehre : V V Typically I lly R in figure OUT REF ADJ REF ADJ = + + × 1 125 2 2 1 2 1 2 = . =50 uA Typica & R as shown Figure 1 - Simplified block diagram of the US1175 Vout 1175app2-1.0 R1 R2 Vin Vctrl Vref IAdj = 50uA US1175 Vsense Adj Vout Vctrl Vin Vctrl Vin Vsense 1175blk1-1.0 Adj Vout THERMAL SHUTDOWN CURRENT LIMIT 1.25V + + |
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