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LT1054L Datasheet(PDF) 6 Page - Linear Technology |
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LT1054L Datasheet(HTML) 6 Page - Linear Technology |
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6 / 16 page  6 LT1054/LT1054L PIN FUNCTIONS from being pulled above the ground pin (Pin 3) during start-up. Any small, general purpose transistor such as 2N2222 or 2N2219 can be used. RX should be chosen to provide enough base drive to the external transistor so that it is saturated under nominal output voltage and maximum output current conditions. In some cases an N-channel enhancement mode MOSFET can be used in place of the transistor. RX ≤ ( |VOUT|) β IOUT – + LOAD CIN COUT LT1054 • F01 IL V+ RX LT1054 FB/SHDN CAP+ GND CAP – V+ OSC VREF VOUT IQ IOUT + VREF (Pin 6): Reference Output. This pin provides a 2.5V reference point for use in LT1054-based regulator circuits. The temperature coefficient of the reference voltage has been adjusted so that the temperature coefficient of the regulated output voltage is close to zero. This requires the reference output to have a positive temperature coefficient as can be seen in the typical performance curves. This nonzero drift is necessary to offset a drift term inherent in the internal reference divider and comparator network tied to the feedback pin. The overall result of these drift terms is a regulated output which has a slight positive tempera- ture coefficient at output voltages below 5V and a slight negative TC at output voltages above 5V. Reference output current should be limited, for regulator feedback networks, to approximately 60 µA. The reference pin will draw ≈100µA when shorted to ground and will not affect the internal reference/regulator, so that this pin can also be used as a pull-up for LT1054 circuits that require synchro- nization. OSC (Pin 7): Oscillator Pin. This pin can be used to raise or lower the oscillator frequency or to synchronize the device to an external clock. Internally Pin 7 is connected to the oscillator timing capacitor (Ct ≈ 150pF) which is alternately charged and discharged by current sources of ±7µAsothat the duty cycle is ≈50%. The LT1054 oscillator is designed to run in the frequency band where switching losses are minimized. However the frequency can be raised, lowered, or synchronized to an external system clock if necessary. The frequency can be lowered by adding an external capacitor (C1, Figure 2) from Pin 7 to ground. This will increase the charge and discharge times which lowers the oscillator frequency. The frequency can be increased by adding an external capacitor (C2, Figure 2, in the range of 5pF to 20pF) from Pin 2 to Pin 7. This capacitor will couple charge into CT at the switch transitions, which will shorten the charge and discharge time, raising the oscillator fre- quency. Synchronization can be accomplished by adding an external resistive pull-up from Pin 7 to the reference pin (Pin 6). A 20k pull-up is recommended. An open collector gate or an NPN transistor can then be used to drive the oscillator pin at the external clock frequency as shown in Figure 2. Pulling up Pin 7 to an external voltage is not recommended. For circuits that require both fre- quency synchronization and regulation, an external refer- ence can be used as the reference point for the top of the R1/R2 divider allowing Pin 6 to be used as a pull-up point for Pin 7. Figure 1 V + (Pin 8): Input Supply. The LT1054 alternately charges CIN to the input voltage when CIN is switched in parallel with the input supply and then transfers charge to COUT when CIN is switched in parallel with COUT. Switching occurs at VIN COUT CIN C2 C1 LT1054 • F02 LT1054 FB/SHDN CAP+ GND CAP – V+ OSC VREF VOUT + Figure 2 |
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