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LT3971 Datasheet(PDF) 11 Page - Linear Technology |
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LT3971 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 26 page  LTC3630 11 3630fb APPLICATIONS INFORMATION The basic LTC3630 application circuit is shown on the front page of the data sheet. External component selection is determined by the maximum load current requirement and begins with the selection of the peak current programming resistor, RISET.TheinductorvalueLcanthenbedetermined, followed by capacitors CIN and COUT. Peak Current Resistor Selection The peak current comparator has a guaranteed maximum current limit of 1A (1.2A typical), which guarantees a maximum average current of 500mA. For applications that demand less current, the peak current threshold can be reduced to as little as 100mA (120mA typical). This lower peak current allows the use of lower value, smaller components (input capacitor, output capacitor, and induc- tor), resulting in lower input supply ripple and a smaller overall DC/DC converter. The threshold can be easily programmed using a resis- tor (RISET) between the ISET pin and ground. The voltage generated on the ISET pin by RISET and the internal 5μA current source sets the peak current. The voltage on the ISET pin is internally limited within the range of 0.1V to 1.0V. The value of resistor for a particular peak current can be selected by using Figure 2 or the following equation: RISET = IPEAK • 0.2 • 106 where 100mA < IPEAK < 1A. The internal 5μA current source is reduced to 1μA in sleep mode to maximize efficiency and to facilitate a trade-off between efficiency and light load output voltage ripple, as described in the CISET Selection section of the Applica- tions Information. For maximum efficiency, minimize the capacitance on the ISET pin and place the RISET resistor as close to the pin as possible. The typical peak current is internally limited to be within the range of 120mA to 1.2A. Shorting the ISET pin to ground programs the current limit to 120mA, and leaving it float sets the current limit to the maximum value of 1.2A. When selecting this resistor value, be aware that the maximum average output current for this architecture is limited to half of the peak current. Therefore, be sure to select a value that sets the peak current with enough margin to provide adequate load current under all conditions. Selecting the peak current to be 2.2 times greater than the maximum load current is a good starting point for most applications. Inductor Selection The inductor, input voltage, output voltage, and peak cur- rent determine the switching frequency during a burst cycle of the LTC3630. For a given input voltage, output voltage, and peak current, the inductor value sets the switching frequency during a burst cycle when the output is in regulation. Generally, switching between 50kHz and 250kHz yields high efficiency, and 200kHz is a good first choice for many applications. The inductor value can be determined by the following equation: L = VOUT f•IPEAK ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ •1– VOUT VIN ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ The variation in switching frequency during a burst cycle with input voltage and inductance is shown in Figure 3. For lower values of IPEAK, multiply the frequency in Figure 3 by 1.2A/IPEAK. An additional constraint on the inductor value is the LTC3630’s 150ns minimum on-time of the high side switch. Therefore, in order to keep the current in the inductor Figure 2. RISET Selection MAXIMUM LOAD CURRENT (mA) 50 60 180 200 220 150 250 300 350 3630 F02 20 140 100 40 160 0 120 80 100 200 400 450 500 |
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