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LTC1553 数据表(PDF) 13 Page - Linear Technology |
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LTC1553 数据表(HTML) 13 Page - Linear Technology |
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13 / 24 page  13 LTC1553 APPLICATIONS INFORMATION Q1 G1 Q2 0.1 µF LO VOUT 1553 F07 COUT CIN VIN PVCC 1N5248B 18V 1N5817 OPTIONAL FOR VIN > 5V G2 LTC1553 20 1 2 + + Figure 7. Doubling Charge Pump MOSFET Gate Drive Power for the internal MOSFET drivers is supplied by PVCC. This supply must be above the input supply voltage by at least one power MOSFET VGS(ON) for efficient opera- tion. This higher voltage can be supplied with a separate supply, or it can be generated using a simple charge pump as shown in Figure 7. The 84% typical maximum duty cycle ensures sufficient off-time to refresh the charge pump during each cycle. Figure 8 shows a tripling charge pump, which provides additional VGS overdrive to the external MOSFETs. This circuit can be useful for standard threshold MOSFETs which demand a higher turn-on volt- age. An 18V Zener diode (1N5248B) is recommended with tripler charge pump designs to ensure that PVCC never exceeds the LTC1553’s 20V absolute maximum PVCC voltage. This becomes more critical as VIN rises. With VIN = 12V, the doubler circuit of Figure 7 will also exceed the 20V limit. Figure 9 shows an alternate 17V charge pump derived from both the 5V and 12V supplies. If the OUTEN pin is low, G1 and G2 are both held low to prevent output voltage undershoot. As VCC and PVCC power up from a 0V condition, an internal undervoltage lockup circuit prevents G1 and G2 from going high until VCC reaches about 3.5V. If VCC powers up while PVCC is at ground potential, the SS is forced to ground potential internally. SS clamps the COMP pin low and prevents the drivers from turning on. On power-up or recovery from thermal shutdown, the drivers are designed such that G2 is held low until G1 first goes high. Power MOSFETs Two N-channel power MOSFETs are required for most LTC1553 circuits. They should be selected based prima- rily on threshold and on-resistance considerations. The required MOSFET threshold should be determined based on the available power supply voltages and/or the com- plexity of the gate driver charge pump scheme. In 5V input designs where a 12V supply is used to power PVCC, standard MOSFETs with RDS(ON) specified at VGS = 5V or 6V can be used with good results. However, logic level devices will improve efficiency. The current drawn from the 12V supply varies with the MOSFETs used and the LTC1553 operating frequency, but is generally less than 50mA. Figure 9. 17V Charge Pump for VIN = 12V Q1 10 Ω Q2 0.1 µF LO VOUT 1553 F09 COUT VIN 12V CVCC 1N5248B 18V 1N5817 VCC 5V LTC1553 CIN G1 PVCC G2 20 1 2 VCC 5 + + Q1 Q2 0.1 µF 0.1 µF 10 µF LO VOUT 1553 F08 COUT CIN VIN 1N5248B 18V 1N5817 LTC1553 1N5817 1N5817 + G1 PVCC G2 20 1 2 + + Figure 8. Tripling Charge Pump |
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