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LT3958 Datasheet(PDF) 11 Page - Linear Technology |
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LT3958 Datasheet(HTML) 11 Page - Linear Technology |
11 / 28 page LT3958 11 3958f APPLICATIONS INFORMATION Operating Frequency and Synchronization The choice of operating frequency may be determined by on-chip power dissipation (a low switching frequency may be required to ensure IC junction temperature does not exceed 125°C), otherwise it is a trade-off between efficiency and component size. Low frequency operation improves efficiency by reducing gate drive current and MOSFET and diode switching losses. However, lower frequency operation requires a physically larger inductor. Switching frequency also has implications for loop compensation. The LT3958 uses a constant-frequency architecture that can be programmed over a 100kHz to 1000kHz range with a single external resistor from the RT pin to ground, as shown in Figure 1. The RT pin must have an external resistor to SGND for proper operation of the LT3958. A table for selecting the value of RT for a given operating frequency is shown in Table 1. Table 1. Timing Resistor (RT) Value SWITCHING FREQUENCY (kHz) RT (kΩ) 100 140 200 63.4 300 41.2 400 30.9 500 24.3 600 19.6 700 16.5 800 14 900 12.1 1000 10.5 The operating frequency of the LT3958 can be synchro- nized to an external clock source. By providing a digital clock signal into the SYNC pin, the LT3958 will operate at the SYNC clock frequency. The LT3958 detects the ris- ing edge of each Sync clock cycle. If this feature is used, an RT resistor should be chosen to program a switching frequency 20% slower than SYNC pulse frequency. Tie the SYNC pin to SGND if this feature is not used. It is recommended that the Sync input clock has a minimum pulse width of 200ns. Duty Cycle Consideration Switching duty cycle is a key variable defining converter operation. As such, its limits must be considered. Minimum on-time is the smallest time duration that the LT3958 is capable of turning on the power MOSFET. This time is generally about 250ns (typical) (see Minimum On-Time in the Electrical Characteristics table). In each switching cycle, the LT3958 keeps the power switch off for at least 200ns (typical) (see Minimum Off-Time in the Electrical Characteristics table). The minimum on-time, minimum off-time and the switching frequency define the minimum and maximum switching duty cycles a converter is able to generate: Minimum duty cycle = minimum on-time • frequency Maximum duty cycle = 1 – (minimum off-time • frequency) Programming the Output Voltage The output voltage VOUT is set by a resistor divider, as shown in Figure 1. The positive and negative VOUT are set by the following equations: VOUT,POSITIVE =1.6V • 1+ R2 R1 ⎛ ⎝⎜ ⎞ ⎠⎟ VOUT,NEGATIVE = –0.8V • 1+ R2 R1 ⎛ ⎝⎜ ⎞ ⎠⎟ The resistors R1 and R2 are typically chosen so that the error caused by the current flowing into the FBX pin during normal operation is less than 1% (this translates to a maximum value of R1 at about 158k). |
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