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LTC3404 Datasheet(PDF) 8 Page - Linear Technology |
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LTC3404 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LTC3404 Figure 1. Maximum Output Current vs Input Voltage Figure 2. Maximum Inductor Peak Current vs Duty Cycle Dropout Operation When the input supply voltage decreases toward the output voltage, the duty cycle increases toward the maxi- mum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the internal P-channel MOSFET and the inductor. Low Supply Operation The LTC3404 is designed to operate down to an input supply voltage of 2.65V although the maximum allowable output current is reduced at this low voltage. Figure 1 shows the reduction in the maximum output current as a function of input voltage for various output voltages. Another important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases. Therefore, the user should calculate the power dissipation when the LTC3404 is used at 100% duty cycle with a low input voltage (see Thermal Considerations in the Applications Information section). Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant fre- quency architectures by preventing subharmonic oscilla- tions at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. As a result, the maximum inductor peak current is reduced for duty cycles > 40%. This is shown in the decrease of the inductor peak current as a function of duty cycle graph in Figure 2. DUTY CYCLE (%) 0 1100 1000 900 800 700 600 80 3404 F02 20 40 60 100 VIN = 3.3V OPERATIO APPLICATIO S I FOR ATIO The basic LTC3404 application circuit is shown on the first page. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. Inductor Value Calculation The inductor selection will depend on the operating fre- quency of the LTC3404. The internal nominal frequency is 1.4MHz, but can be externally synchronized from 1MHz to 1.7MHz. The operating frequency and inductor selection are inter- related in that higher operating frequencies allow the use of smaller inductor and capacitor values. However, oper- ating at a higher frequency generally results in lower efficiency because of increased internal gate charge losses. The inductor value has a direct effect on ripple current. The ripple current ∆IL decreases with higher inductance or frequency and increases with higher VIN or VOUT. SUPPLY VOLTAGE (V) 2.5 1200 1000 800 600 400 200 0 3.5 4.5 5.5 6.5 3404 • F01 7.5 L = 4.7 µH VOUT = 3.3V VOUT = 2.5V VOUT = 1.5V |
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