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LTC3251-1.5 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3251-1.5 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC3251/ LTC3251-1.2/LTC3251-1.5 32511215fb OPERATIO (Refer to Block Diagram) Figure 3 capacitor selection, etc.), the part’s burst current is set by the burst threshold and hysteresis. This means that the VOUT ripple voltage in Burst Mode operation will be fixed and is typically 15mV with a 10 µF output capacitor. Ultralow Current Super Burst Mode Operation To further optimize the supply current for low output current requirements, a Super Burst mode operaton is included in the LTC3251 family of parts. This mode is very similar to Burst Mode operation, but much of the internal circuitry and switch is shut down to further reduce supply current. In Super Burst mode operation an internal hyster- etic comparator is used to enable/disable charge transfer. The hysteresis of the comparator and the amount of current deliverable to the output are limited to keep output ripple low. The VOUT ripple voltage in Super Burst mode operation is typically 35mV with a 10 µF output capacitor. The LTC3251 family can deliver 40mA of current in Super Burst mode operation but does not switch to continuous mode. The MODE pin of the LTC3251-1.2 and LTC3251- 1.5 has no effect on operation in super-burst mode. VOUT Capacitor Selection The style and value of capacitors used with the LTC3251 family determine several important parameters such as regulator control loop stability, output ripple and charge pump strength. The dual phase nature of the LTC3251 family minimizes output noise significantly but not completely. What small ripple that does exist is controlled by the value of COUT directly. Increasing the size of COUT will proportionately reduce the output ripple. The ESR (equivalent series resistance) of COUT plays the dominant role in output noise. When a part switches between clock phases there is a period where all switches are turned off. This “blanking period” shows up as a spike at the output and is a direct function of the output current times the ESR value. To reduce output noise and ripple, it is suggested that a low ESR (<0.08 Ω) ceramic capacitor be used for COUT. Tanta- lum and aluminum capacitors are not recommended be- cause of their high ESR. Both the style and value of COUT can significantly affect the stability of the LTC3251 family. As shown in the Block Diagram, the LTC3251 family uses a control loop to adjust the strength of the charge pump to match the current required at the output. The error signal of this loop is stored directly on the output charge storage capacitor. Thus the charge storage capacitor also serves to form the dominant pole for the control loop. The desired output voltage also affects stability. As the divider ratio (RA/RB) drops, the effective closed-loop gain increases, thus re- quiring a larger output capacitor for stability. Figure 3 shows the suggested output capacitor for optimal tran- sient response. The value of the output capacitance should not drop below the minimum capacitance line to prevent excessive ringing or instability. (see Ceramic Capacitor Selection Guidelines section). Likewise excessive ESR on the output capacitor will tend to degrade the loop stability. The closed loop output impedance of the LTC3251 is approximately: R V V O OUT ≅Ω 0 045 08 .• . For example, with the output programmed to 1.5V, the RO is 0.085 Ω, which produces a 40mV output change for a 500mA load current step. For stability and good load transient response, it is important for the output capacitor to have 0.08 Ω or less of ESR. Ceramic capacitors typically have exceptional ESR, and combined with a tight board layout, should yield excellent stability and load transient performance. VOUT (V) 0.9 8 14 15 16 1.1 1.3 1.4 3251 F03 6 12 10 7 13 4 5 11 9 1.0 1.2 1.5 1.6 OPTIMUM CAPACITANCE MINIMUM CAPACITANCE |
Similar Part No. - LTC3251-1.5_15 |
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Similar Description - LTC3251-1.5_15 |
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