Datasheet LTC1772B (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | Constant Frequency Current Mode Step-Down DC/DC Controller in SOT-23 |
| Pages / Page | 16 / 7 — OPERATION. (Refer to Functional Diagram). Overvoltage Protection. … |
| File Format / Size | PDF / 226 Kb |
| Document Language | English |
OPERATION. (Refer to Functional Diagram). Overvoltage Protection. Undervoltage Lockout
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LTC1772B
OPERATION (Refer to Functional Diagram)
that the external P-channel MOSFET will remain on for quency will gradually increase to its designed rate when more than one oscillator cycle since the inductor current the feedback voltage again approaches 0.8V. has not ramped up to the threshold set by EAMP. Further reduction in input supply voltage will eventually cause the
Overvoltage Protection
P-channel MOSFET to be turned on 100%, i.e., DC. The As a further protection, the overvoltage comparator in output voltage will then be determined by the input volt- the LTC1772B will turn the external MOSFET off when age minus the voltage drop across the MOSFET, the sense the feedback voltage has risen 7.5% above the reference resistor and the inductor. voltage of 0.8V. This comparator has a typical hysteresis of 20mV.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
Slope Compensation and Inductor’s Peak Current
input voltage levels, an undervoltage lockout is incorpo- The inductor’s peak current is determined by: rated into the LTC1772B. When the input supply voltage drops below approximately 2.0V, the P-channel MOSFET V – . 0 85 I ITH PK = and all circuitry is turned off except the undervoltage block, 10 RSENSE ( ) which draws only several microamperes. when the LTC1772B is operating below 40% duty
Short-Circuit Protection
cycle. However, once the duty cycle exceeds 40%, slope When the output is shorted to ground, the frequency of compensation begins and effectively reduces the peak the oscillator will be reduced to about 120kHz. This lower inductor current. The amount of reduction is given by the frequency allows the inductor current to safely discharge, curves in Figure 3. thereby preventing current runaway. The oscillator’s fre- 110 100 90 (%) 80 70 OUT(MAX) 60 /I 50 OUT IRIPPLE = 0.4IPK 40 AT 5% DUTY CYCLE SF = I I 30 RIPPLE = 0.2IPK AT 5% DUTY CYCLE 20 VIN = 4.2V 10 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 1772 F03
Figure 3. Maximum Output Current vs Duty Cycle
1772bfa 7
OPERATION (Refer to Functional Diagram)
that the external P-channel MOSFET will remain on for quency will gradually increase to its designed rate when more than one oscillator cycle since the inductor current the feedback voltage again approaches 0.8V. has not ramped up to the threshold set by EAMP. Further reduction in input supply voltage will eventually cause the
Overvoltage Protection
P-channel MOSFET to be turned on 100%, i.e., DC. The As a further protection, the overvoltage comparator in output voltage will then be determined by the input volt- the LTC1772B will turn the external MOSFET off when age minus the voltage drop across the MOSFET, the sense the feedback voltage has risen 7.5% above the reference resistor and the inductor. voltage of 0.8V. This comparator has a typical hysteresis of 20mV.
Undervoltage Lockout
To prevent operation of the P-channel MOSFET below safe
Slope Compensation and Inductor’s Peak Current
input voltage levels, an undervoltage lockout is incorpo- The inductor’s peak current is determined by: rated into the LTC1772B. When the input supply voltage drops below approximately 2.0V, the P-channel MOSFET V – . 0 85 I ITH PK = and all circuitry is turned off except the undervoltage block, 10 RSENSE ( ) which draws only several microamperes. when the LTC1772B is operating below 40% duty
Short-Circuit Protection
cycle. However, once the duty cycle exceeds 40%, slope When the output is shorted to ground, the frequency of compensation begins and effectively reduces the peak the oscillator will be reduced to about 120kHz. This lower inductor current. The amount of reduction is given by the frequency allows the inductor current to safely discharge, curves in Figure 3. thereby preventing current runaway. The oscillator’s fre- 110 100 90 (%) 80 70 OUT(MAX) 60 /I 50 OUT IRIPPLE = 0.4IPK 40 AT 5% DUTY CYCLE SF = I I 30 RIPPLE = 0.2IPK AT 5% DUTY CYCLE 20 VIN = 4.2V 10 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) 1772 F03
Figure 3. Maximum Output Current vs Duty Cycle
1772bfa 7
