Datasheet LTC1622 (Analog Devices) - 6

LTC1622
U OPERATIO (Refer to Functional Diagram)
the ITH pin will drop. When the ITH voltage goes below
Short-Circuit Protection
0.12V, the sleep signal goes high, turning off the external When the output is shorted to ground, the frequency of the MOSFET. The sleep signal goes low when the ITH voltage oscillator will be reduced to about 110kHz. This lower rises above 0.22V and the LTC1622 resumes normal frequency allows the inductor current to safely discharge, operation. The next oscillator cycle will turn the external thereby preventing current runaway. The oscillator’s fre- MOSFET on and the switching cycle repeats. quency will gradually increase to its nominal value when the feedback voltage increases above 0.65V. Note that
Frequency Synchronization
synchronization is inhibited until the feedback voltage The LTC1622 can be externally driven by a TTL/CMOS goes above 0.3V. compatible clock signal up to 750kHz. Do not synchronize the LTC1622 below its maximum default operating fre-
Overvoltage Protection
quency of 625kHz as this may cause abnormal operation As a further protection, the overvoltage comparator in the and an undesired frequency spectrum. The LTC1622 is LTC1622 will turn the external MOSFET off when the synchronized to the rising edge of the clock. The external feedback voltage has risen 16% above the reference clock pulse width must be at least 100ns and not more voltage of 0.8V. This comparator has a typical hysteresis than the period minus 200ns. of 35mV. Synchronization is inhibited when the feedback voltage is below 0.3V. This is to prevent inductor current buildup
Slope Compensation and Peak Inductor Current
under short-circuit conditions. Burst Mode operation is The inductor’s peak current is determined by: deactivated when the LTC1622 is externally driven by a clock. V I ITH PK =
Dropout Operation
10 R ( ) SENSE When the input supply voltage decreases towards the when the LTC1622 is operating below 40% duty cycle. output voltage, the rate of change of inductor current However, once the duty cycle exceeds 40%, slope com- during the ON cycle decreases. This reduction means that pensation begins and effectively reduces the peak induc- the P-channel MOSFET will remain on for more than one tor current. The amount of reduction is given by the curves oscillator cycle since the inductor current has not ramped in Figure 2. up to the threshold set by EA. Further reduction in input supply voltage will eventually cause the P-channel MOSFET 110 to be turned on 100%, i.e., DC. The output voltage will then 100 be determined by the input voltage minus the voltage drop 90 across the MOSFET, the sense resistor and the inductor. 80 (%) 70
Undervoltage Lockout
OUT(MAX) 60 /I 50 IRIPPLE = 0.4IPK To prevent operation of the P-channel MOSFET below safe OUT AT 5% DUTY CYCLE 40 I input voltage levels, an undervoltage lockout is incorpo- SF = I RIPPLE = 0.2IPK AT 5% DUTY CYCLE 30 rated into the LTC1622. When the input supply voltage V 20 IN = 4.2V drops below 2V, the P-channel MOSFET and all circuitry is UNSYNC 10 turned off except the undervoltage block, which draws 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%) only several microamperes. 1622 F02
Figure 2. Maximum Output Current vs Duty Cycle
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