Datasheet LTC1707 (Analog Devices) - 7

LTC1707
U OPERATIO
Frequency synchronization is inhibited when the feedback switch is on continuously. Hence, the I2R loss is due voltage VFB is below 0.6V. This prevents the external clock mainly to the RDS(ON) of the P-channel MOSFET. See from interfering with the frequency foldback for short- Efficiency Considerations in the Applications Information circuit protection. section.
Dropout Operation
Below VIN = 4V, the output current must be derated as shown in Figures 2a and 2b. For applications that require When the input supply voltage decreases toward the out- 500mA below VIN = 4V, select the LTC1627. put voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the 1200 main switch to remain on for more than one cycle until it 1000 VOUT = 1.8V reaches 100% duty cycle. The output voltage will then be VOUT = 1.5V determined by the input voltage minus the voltage drop 800 across the P-channel MOSFET and the inductor. VOUT = 5V 600 In Burst Mode operation or pulse skipping mode operation VOUT = 3.3V 400 with the output lightly loaded, the LTC1707 transitions OUTPUT CURRENT (mA) VOUT = 2.5V through continuous mode as it enters dropout. 200 VOUT = 2.9V TJ = 25°C L = 15µH EXT SYNC AT 400kHz 0
Undervoltage Lockout
2.5 3.5 4.5 5.5 6.5 7.5 8.5 INPUT VOLTAGE (V) A precision undervoltage lockout shuts down the LTC1707 1707 F02b when VIN drops below 2.7V, making it ideal for single
Figure 2b. Maximum Output Current
lithium-ion battery applications. In lockout, the LTC1707
vs Input Voltage (Synchronized)
draws only several microamperes, which is low enough to prevent deep discharge and possible damage to the lithium-
Slope Compensation and Inductor Peak Current
ion battery nearing its end of charge. A 100mV hysteresis Slope compensation provides stability by preventing sub- ensures reliable operation with noisy input supplies. harmonic oscillations. It works by internally adding a ramp to the inductor current signal at duty cycles in excess of
Low Supply Operation
40%. As a result, the maximum inductor peak current is The LTC1707 is designed to operate down to a 2.85V input lower for VOUT/VIN > 0.4 than when VOUT/VIN < 0.4. See the voltage. At this voltage the converter is most likely to be inductor peak current as a function of duty cycle graph in running at high duty cycles or in dropout where the main Figure 3. The worst-case peak current reduction occurs 1200 1000 1000 VOUT = 1.8V VOUT = 1.5V WITHOUT 900 800 EXTERNAL CLOCK SYNC VOUT = 5V 600 800 WORST-CASE EXTERNAL 400 VOUT = 3.3V 700 CLOCK SYNC OUTPUT CURRENT (mA) VOUT = 2.5V 200 VOUT = 2.9V TJ = 25°C 600 L = 15µH 0 V 2.5 3.5 4.5 5.5 MAXIMUM INDUCTOR PEAK CURRENT (mA) 6.5 7.5 8.5 IN = 4V 500 INPUT VOLTAGE (V) 0 10 20 30 40 50 60 70 80 90 100 1707 F02a DUTY CYCLE (%)
Figure 2a. Maximum Output Current
1707 F03
vs Input Voltage (Unsynchronized) Figure 3. Maximum Inductor Peak Current vs Duty Cycle
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