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LTC3610EWP-PBF Datasheet(PDF) 11 Page - Linear Technology |
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LTC3610EWP-PBF Datasheet(HTML) 11 Page - Linear Technology |
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11 / 24 page  LTC3610 3610ff applications inForMation Figure1a.SwitchingFrequencyvsRON(VON=0V) Figure1b.SwitchingFrequencyvsRON(VON=INTVCC) Because the voltage at the ION pin is about 0.7V, the cur- rent into this pin is not exactly inversely proportional to VIN, especially in applications with lower input voltages. To correct for this error, an additional resistor RON2 con- nected from the IONpintothe5VINTVCCsupplywillfurther stabilize the frequency. R V V R ON ON 2 5 0 7 = . Changes in the load current magnitude will also cause frequency shift. Parasitic resistance in the MOSFET switches and inductor reduce the effective voltage across the inductance, resulting in increased duty cycle as the load current increases. By lengthening the on-time slightly as current increases, constant frequency operation can be maintained. This is accomplished with a resistive divider from the ITH pin to the VON pin and VOUT. The values required will depend on the parasitic resistances in the specific application. A good starting point is to feed about 25% of the voltage change at the ITH pin to the VON pin as shown in Figure 2a. Place capacitance on the VON pin to filter out the ITH variations at the switching frequency. The resistor load on ITH reduces the DC gain of the error amp and degrades load regulation, which can be avoided by using the PNP emitter follower of Figure 2b. MinimumOff-TimeandDropoutOperation The minimum off-time, tOFF(MIN), is the smallest amount of time that the LTC3610 is capable of turning on the bot- tom MOSFET, tripping the current comparator and turning the MOSFET back off. This time is generally about 250ns. The minimum off-time limit imposes a maximum duty cycle of tON/(tON + tOFF(MIN)). If the maximum duty cycle is reached, due to a dropping input voltage for example, then the output will drop out of regulation. The minimum input voltage to avoid dropout is: V V t t t IN MIN OUT ON OFF MIN ON ( ) ( ) = + A plot of maximum duty cycle vs frequency is shown in Figure 3. SettingtheOutputVoltage The LTC3611 develops a 0.6V reference voltage between the feedback pin, VFB, and the signal ground as shown in Figure 6. The output voltage is set by a resistive divider according to the following formula: VOUT = 0.6V 1+ R2 R1 ⎛ ⎝⎜ ⎞ ⎠⎟ To improve the frequency response, a feedforward capaci- tor C1 may also be used. Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW trace. RON (k) 100 100 1000 1000 10000 3610 F01a VOUT = 3.3V VOUT = 1.5V VOUT = 2.5V RON (k) 100 100 1000 1000 10000 3610 F01b VOUT = 3.3V VOUT = 12V VOUT = 5V |
Similar Part No. - LTC3610EWP-PBF |
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Similar Description - LTC3610EWP-PBF |
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