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LTC1174HVCS8 Datasheet(PDF) 9 Page - Linear Technology |
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LTC1174HVCS8 Datasheet(HTML) 9 Page - Linear Technology |
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9 / 20 page  9 LTC1174 LTC1174-3.3/LTC1174-5 1174fe Catch Diode Selection The catch diode carries load current during the off-time. The average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the diode conducts most of the time. As VIN approaches VOUT the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is short-circuited. Under this condition the diode must safely handleIPEAKatcloseto100%dutycycle.Afastswitchingdiode must also be used to optimize efficiency. Schottky diodes are a good choice for low forward drop and fast switching times. Most LTC1174 circuits will be well served by either a 1N5818, a MBRS140T3 or a MBR0520L Schottky diode. Short-Circuit Protection The LTC1174 is protected from output short by its internal current limit. Depending on the condition of IPGM pin, the limit is either set to 340mA or 600mA. In addition, the off- time of the switch is increased to allow the inductor’s current to decay far enough to prevent any current build-up (see Figure 2). APPLICATIO S I FOR ATIO compared with a 1.25V reference voltage. With the current going into Pin 3 being negligible, the following expression is used for setting the trip limit: V R R LBTRIP =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 125 1 4 3 . When the LTC1174 is shut down, the low-battery detector is inactive. IPGM = VIN IPGM = 0 GND L = 100 µH VIN = 13.5V 20 µs/DIV 1174 F02 Figure 2. Inductor's Current with Output Shorted Low-Battery Detector The low-battery indicator senses the input voltage through an external resistive divider. This divided voltage connects to the “–” input of a voltage comparator (Pin 3) which is Figure 3. Low-Battery Comparator LTC1174 – + 1.25V REFERENCE R4 R3 3 VIN 1174 F03 LTC1174 Adjustable/Low Noise Applications The LTC1174 develops a 1.25V reference voltage between the feedback (Pin 1) terminal and ground (see Figure 4). By selecting resistor R1, a constant current is caused to flow through R1 and R2 to set the overall output voltage. The regulated output voltage is determined by: V R R OUT =+ ⎛ ⎝⎜ ⎞ ⎠⎟ 125 1 2 1 . For most applications, a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested across R1 located close to the LTC1174. Alternatively, a capacitor across R2 can be used to increase the switching frequency for low noise operation. Inverting Applications The LTC1174 can easily be set up for a negative output voltage. If – 5V is desired, the LTC1174-5 is ideal for this application as it requires the least components. Figure 5 shows the schematic for this application. Note that the |
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