Datasheet LTC1174, LTC1174-3.3, LTC1174-5 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | High Efficiency Step-Down and Inverting DC/DC Converter |
| Pages / Page | 20 / 9 — APPLICATIO S I FOR ATIO. Catch Diode Selection. Short-Circuit Protection. … |
| File Format / Size | PDF / 263 Kb |
| Document Language | English |
APPLICATIO S I FOR ATIO. Catch Diode Selection. Short-Circuit Protection. Figure 3. Low-Battery Comparator
Model Line for this Datasheet
- LTC1174CN8#PBF LTC1174CN8-3.3#PBF LTC1174CN8-5#PBF LTC1174CS8#PBF LTC1174CS8#TRPBF LTC1174CS8-3.3#PBF LTC1174CS8-3.3#TRPBF LTC1174CS8-5#PBF LTC1174CS8-5#TRPBF LTC1174HVCN8#PBF LTC1174HVCN8-3.3#PBF LTC1174HVCN8-5#PBF LTC1174HVCS8#PBF LTC1174HVCS8#TRPBF LTC1174HVCS8-3.3#PBF LTC1174HVCS8-3.3#TRPBF LTC1174HVCS8-5#PBF LTC1174HVCS8-5#TRPBF LTC1174HVIS8#PBF LTC1174HVIS8#TRPBF LTC1174IN8#PBF LTC1174IS8#PBF LTC1174IS8#TRPBF
Text Version of Document
LTC1174 LTC1174-3.3/LTC1174-5
U U W U APPLICATIO S I FOR ATIO Catch Diode Selection
compared with a 1.25V reference voltage. With the current going into Pin 3 being negligible, the following expression The catch diode carries load current during the off-time. The is used for setting the trip limit: average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the ⎛ R4⎞ diode conducts most of the time. As V V 1 2 . 5 1 IN approaches VOUT LBTRIP = + ⎝⎜ R3 ⎠⎟ the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is When the LTC1174 is shut down, the low-battery detector short-circuited. Under this condition the diode must safely is inactive. handle IPEAK at close to 100% duty cycle. A fast switching diode V must also be used to optimize efficiency. Schottky diodes are IN LTC1174 a good choice for low forward drop and fast switching times. R4 Most LTC1174 circuits will be well served by either a 1N5818, 3 – a MBRS140T3 or a MBR0520L Schottky diode. + R3 1.25V
Short-Circuit Protection
REFERENCE 1174 F03 The LTC1174 is protected from output short by its internal current limit. Depending on the condition of I
Figure 3. Low-Battery Comparator
PGM 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
LTC1174 Adjustable/Low Noise Applications
current to decay far enough to prevent any current build-up The LTC1174 develops a 1.25V reference voltage between (see Figure 2). 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: IPGM = VIN ⎛ R2⎞ VOUT = 1.25 1 + ⎝⎜ R ⎠⎟ 1 IPGM = 0 For most applications, a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested GND across R1 located close to the LTC1174. Alternatively, a 1174 F02 L = 100µH 20µs/DIV capacitor across R2 can be used to increase the switching VIN = 13.5V frequency for low noise operation.
Figure 2. Inductor's Current with Output Shorted Inverting Applications Low-Battery Detector
The LTC1174 can easily be set up for a negative output The low-battery indicator senses the input voltage through voltage. If – 5V is desired, the LTC1174-5 is ideal for this an external resistive divider. This divided voltage connects application as it requires the least components. Figure 5 to the “–” input of a voltage comparator (Pin 3) which is shows the schematic for this application. Note that the 1174fe 9
U U W U APPLICATIO S I FOR ATIO Catch Diode Selection
compared with a 1.25V reference voltage. With the current going into Pin 3 being negligible, the following expression The catch diode carries load current during the off-time. The is used for setting the trip limit: average diode current is therefore dependent on the P-channel switch duty cycle. At high input voltages the ⎛ R4⎞ diode conducts most of the time. As V V 1 2 . 5 1 IN approaches VOUT LBTRIP = + ⎝⎜ R3 ⎠⎟ the diode conducts only a small fraction of the time. The most stressful condition for the diode is when the output is When the LTC1174 is shut down, the low-battery detector short-circuited. Under this condition the diode must safely is inactive. handle IPEAK at close to 100% duty cycle. A fast switching diode V must also be used to optimize efficiency. Schottky diodes are IN LTC1174 a good choice for low forward drop and fast switching times. R4 Most LTC1174 circuits will be well served by either a 1N5818, 3 – a MBRS140T3 or a MBR0520L Schottky diode. + R3 1.25V
Short-Circuit Protection
REFERENCE 1174 F03 The LTC1174 is protected from output short by its internal current limit. Depending on the condition of I
Figure 3. Low-Battery Comparator
PGM 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
LTC1174 Adjustable/Low Noise Applications
current to decay far enough to prevent any current build-up The LTC1174 develops a 1.25V reference voltage between (see Figure 2). 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: IPGM = VIN ⎛ R2⎞ VOUT = 1.25 1 + ⎝⎜ R ⎠⎟ 1 IPGM = 0 For most applications, a 30k resistor is suggested for R1. To prevent stray pickup, a 100pF capacitor is suggested GND across R1 located close to the LTC1174. Alternatively, a 1174 F02 L = 100µH 20µs/DIV capacitor across R2 can be used to increase the switching VIN = 13.5V frequency for low noise operation.
Figure 2. Inductor's Current with Output Shorted Inverting Applications Low-Battery Detector
The LTC1174 can easily be set up for a negative output The low-battery indicator senses the input voltage through voltage. If – 5V is desired, the LTC1174-5 is ideal for this an external resistive divider. This divided voltage connects application as it requires the least components. Figure 5 to the “–” input of a voltage comparator (Pin 3) which is shows the schematic for this application. Note that the 1174fe 9
