Datasheet LTC1773 (Analog Devices) - 7

LTC1773
U OPERATIO (Refer to Functional Diagram)
and forces the main switch to stay off for the same number
Low Supply Operation
of cycles. Increasing the output load current slightly, The LTC1773 is designed to operate down to a 2.65V above the minimum required for discontinuous conduc- supply voltage. For proper operation at this low input tion mode, allows constant frequency PWM. voltage, sub-logic level MOSFETs are required. When the Frequency synchronization is inhibited when the feedback value of the output voltage is very close to the input voltage, VFB, is below 0.6V. This prevents the external voltage, the converter is running at high duty cycles or in clock from interfering with the frequency foldback for dropout where the main switch is on continuously. See short-circuit protection. Efficiency Considerations in the Applications Information section.
Dropout Operation Slope Compensation and Inductor Peak Current
When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maxi- Slope compensation provides stability by preventing mum on-time. Further reduction of the supply voltage subharmonic oscillations. It works by internally adding a forces the main switch to remain on for more than one ramp to the inductor current signal at duty cycles in excess cycle until it reaches 100% duty cycle. The output voltage of 30%. This causes the internal current comparator to trip will then be determined by the input voltage minus the IR earlier. The ITH clamp level is also reached earlier than voltage drop across the external P-channel MOSFET, conditions in which the duty cycle is below 30%. As a sense resistor, and the inductor. result, the maximum inductor peak current is lower for VOUT/VIN > 0.3 than when VOUT/VIN < 0.3.
Undervoltage Lockout
To compensate for this loss in maximum inductor peak A precision undervoltage lockout shuts down the LTC1773 current during high duty cycles, the LTC1773 uses a when VIN drops below 2.5V, making it ideal for single patent pending scheme that raises the ITH clamp level lithium-ion battery applications. In shutdown, the LTC1773 (proportional to the amount of slope compensation) when draws only several microamperes, which is low enough to duty cycle is above 30%. prevent deep discharge and possible damage to the lithium- ion battery that’s nearing its end of charge. A 150mV hysteresis ensures reliable operation with noisy supplies.
U U W U APPLICATIONS INFORMATION
The basic LTC1773 application circuit is shown in average output current IMAX equal to the peak value less Figure 1. External component selection is driven by the half the peak-to-peak ripple current ∆IL. load requirement and begins with the selection of RSENSE. Allowing a margin for variations in the LTC1773 and Once RSENSE is known, L can be chosen, followed by the external component values yields: external power MOSFETs. Finally, CIN and COUT are se- lected. RSENSE = 70mV/IMAX
RSENSE Selection for Output Current Inductor Value Calculation
RSENSE is chosen based on the required output current. The inductor selection will depend on the operating fre- The LTC1773 current comparator has a maximum thresh- quency of the LTC1773. The internal preset frequency is old of 100mV/RSENSE. The current comparator threshold 550kHz, but can be externally synchronized up to 750kHz. sets the peak of the inductor current, yielding a maximum 1773fb 7