Datasheet LT4351 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | MOSFET Diode-OR Controller |
| Pages / Page | 20 / 9 — OPERATION. APPLICATIONS INFORMATION. Setting Fault Thresholds. Figure 1. … |
| File Format / Size | PDF / 346 Kb |
| Document Language | English |
OPERATION. APPLICATIONS INFORMATION. Setting Fault Thresholds. Figure 1. Figure 2. Figure 3. Figure 4
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LT4351
OPERATION
The on-chip boost regulator uses a constant off-time The boost regulator regulates VDD to approximately 10.7V control scheme. When VDD is below the regulation trip above VIN When VDD is above this level, the SW transistor voltage, the switch turns on after a 600ns off-time. When turn-on is disabled. When VDD falls below this level by the the switch turns on current ramps up in the inductor until hysteresis level, the SW transistor is allowed to turn on. the current limit is reached (450mA). The switch turns There is approximately 0.15V of hysteresis. off and the inductor’s current flows through the external diode to charge up the VDD capacitor. If VDD is still too low, the switch turns on again after a fixed off-time of 600ns.
APPLICATIONS INFORMATION Setting Fault Thresholds
age at the input. VUV is the part undervoltage trip point The gate drive amplifier implements the ideal diode func- (0.3V) and IHYSTUV is the undervoltage hysteresis current tion. The fault comparators (UV and OV) prevent out of (10µA). See Figure 1. range input voltages from affecting the output by disabling The divider on the OV pin is a straightforward resistive the amplifier during these conditions. Think of the UV and divider (Figure 2): OV as gating the ideal diode function, something a regular diode cannot do. OV R FAULT B = – 1 A resistive divider from V VOV RA IN to UV and one from VIN to OV are the usual way of setting the FAULT thresholds. For UV 0.3V R the resistor values are set by: A = RA,RB Divider Current UV where OVFAULT is the desired overvoltage trip point at the R2 = HYST I input and VOV is the OV pin threshold (0.3V). The OV pin UVHYST has 7mV of voltage hysteresis at room. V R1= UV • R2 It is possible to do both dividers together using only three UVFAULT – VUV resistors though with more interdependence in compo- where UVHYST is the desired undervoltage hysteresis at nents (Figure 3). The input bias current for UV and OV is the input. UVFAULT is the desired undervoltage trip volt- less than 200nA, so keep resistor values less than 10k. VIN VIN VIN VIN VIN R2 R2 R3 R2B UV V R UV UV VUV B UV 300mV 300mV OV VOV 300mV R2 R2A C1 R1 IHYS R1 IHYS RA OV UV 10µA 10µA R1 R1 4351 F02 4351 F01 4351 F03 4351 F04 UV TURNING ON UV TURNING OFF
Figure 1 Figure 2 Figure 3 Figure 4
4351fd 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts
OPERATION
The on-chip boost regulator uses a constant off-time The boost regulator regulates VDD to approximately 10.7V control scheme. When VDD is below the regulation trip above VIN When VDD is above this level, the SW transistor voltage, the switch turns on after a 600ns off-time. When turn-on is disabled. When VDD falls below this level by the the switch turns on current ramps up in the inductor until hysteresis level, the SW transistor is allowed to turn on. the current limit is reached (450mA). The switch turns There is approximately 0.15V of hysteresis. off and the inductor’s current flows through the external diode to charge up the VDD capacitor. If VDD is still too low, the switch turns on again after a fixed off-time of 600ns.
APPLICATIONS INFORMATION Setting Fault Thresholds
age at the input. VUV is the part undervoltage trip point The gate drive amplifier implements the ideal diode func- (0.3V) and IHYSTUV is the undervoltage hysteresis current tion. The fault comparators (UV and OV) prevent out of (10µA). See Figure 1. range input voltages from affecting the output by disabling The divider on the OV pin is a straightforward resistive the amplifier during these conditions. Think of the UV and divider (Figure 2): OV as gating the ideal diode function, something a regular diode cannot do. OV R FAULT B = – 1 A resistive divider from V VOV RA IN to UV and one from VIN to OV are the usual way of setting the FAULT thresholds. For UV 0.3V R the resistor values are set by: A = RA,RB Divider Current UV where OVFAULT is the desired overvoltage trip point at the R2 = HYST I input and VOV is the OV pin threshold (0.3V). The OV pin UVHYST has 7mV of voltage hysteresis at room. V R1= UV • R2 It is possible to do both dividers together using only three UVFAULT – VUV resistors though with more interdependence in compo- where UVHYST is the desired undervoltage hysteresis at nents (Figure 3). The input bias current for UV and OV is the input. UVFAULT is the desired undervoltage trip volt- less than 200nA, so keep resistor values less than 10k. VIN VIN VIN VIN VIN R2 R2 R3 R2B UV V R UV UV VUV B UV 300mV 300mV OV VOV 300mV R2 R2A C1 R1 IHYS R1 IHYS RA OV UV 10µA 10µA R1 R1 4351 F02 4351 F01 4351 F03 4351 F04 UV TURNING ON UV TURNING OFF
Figure 1 Figure 2 Figure 3 Figure 4
4351fd 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Revision History Typical Application Related Parts
