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LTC1704 データシート(PDF) 25 Page - Linear Technology |
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LTC1704 データシート(HTML) 25 Page - Linear Technology |
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25 / 28 page  LTC1704/LTC1704B 25 1704bfa where VBE(QEXT) is base emitter voltage of QEXT and VDROPOUT is the LTC1704 linear regulator controller drop- out voltage. The MJD44H11 from ON Semiconductor has a VBE of around 0.9V at IC = 2A, 25°C and the LTC1704’s VDROPOUT is 1.1V maximum with 30mA of drive current. If the computed minimum VCC is less than the LTC1704 requirement of 3.15V then 3.15V should be used. The minimum VINREG is determined by the VCE saturation voltage of QEXT when it is driven with a base current equal to the maximum REGDR pin drive current. The D44H11 has a saturation voltage of around 0.2V at IC = 2A, 25°C. A typical 1.5V VOUTREG, 2A application will need a mini- mum VCC of 1.5V + 0.9V + 1.1V = 3.5V and a minimum VINREG of 1.5V + 0.2V = 1.7V to operate. If a VOUTREG of 0.8V is needed, the minimum VCC should be 3.15V and the minimum VINREG is 0.8V + 0.2V = 1V. External NPN Pass Transistor The external NPN Pass transistor for the LTC1704 linear regulator supply should be selected based on the follow- ing criteria: 1. Maximum output current 2. DC current gain hFE 3. Total allowable power dissipation 4. Gain bandwidth product fT The NPN transistor must be able to supply the maximum operating current for the linear regulator supply. At the same time, the DC current gain hFE must be large enough such that the pass transistor can supply the maximum load current with 30mA of base current. The transistor must not be subjected to power dissipation higher than the rated value, both during normal operation and over- load conditions. Heat sink can be used to increase the alloweable power dissipation rating. The gain bandwidth product fT of the transistor determines how fast the linear regulator can follow an output load change without losing voltage regulation. The MJD44H11 from ON Semiconductor and SGS- Thomson can be used in the LTC1704 linear regulator supply with current ratings up to 2A. The MJD44H11 from ON Semiconductor can supply 8A of output current and the minimum DC Current Gain hFE is 60 at IC = 2A. The power dissipation rating is 1.75W without heat sink and the gain bandwidth product fT of the MJD44H11 is typi- cally 50MHz. Linear Regulator Supply Current Limit Programming The LTC1704 linear regulator uses an external resistor RREGILM to program the NPN pass transistor base current. This indirectly programs the linear regulator current limit threshold. Figure 13 shows the setup. One end of the resistor RREGILM is connected to an external voltage source VREGON or, alternatively, it can be connected to the VCC pin. The other end of the resistor is connected to the REGILM pin. REGILM is internally regulated to 0.8V. The voltage difference across this resistor generates the REGILM pin input current. This current, together with the internal 1.9 µAcurrentsource,programstheREGDRmaxi- mum output current. The actual linear regulator current limit depends on the pass transistor’s widely distributed DC current gain hFE, which makes this current limit scheme not particularly accurate. Nevertheless, this method re- moves the expensive current sense resistor and with careful design, it is sufficient to protect the external NPN from over damaging. The following equation shows the relationship between RREGILM and the linear regulator current limit threshold ILT: R V I h mA REGILM REGON LT FE = ()( ) –. –. 0 8 2100 75 where VREGON is the pull-up voltage source for RREGILM (see Figure 13). When there is an overload at the linear regulator output, the current limit circuit fires and the output voltage drops. To protect the NPN from excessive heating, the controller APPLICATIO S I FOR ATIO |
同様の部品番号 - LTC1704_15 |
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同様の説明 - LTC1704_15 |
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