LTC3873 APPLICATIONS INFORMATION The circuit in Figure 6 shows a third way to power the ringing on the SW pin disrupts the tiny slope compensa- LTC3873. An external series pre-regulator consisting of tion current out of the pin. It is not recommended to add series pass transistor Q1, zener diode D1 and bias resis- external slope compensation in this case. tor RB brings VCC to at least 7.6V nominal, well above the maximum rated VCC turn-off threshold of 4V. Resistor Output Voltage Programming RSTART momentarily charges the VCC node up to the VCC The output voltage is set by a resistor divider according turn-on threshold, enabling the LTC3873. to the following formula: VIN V ⎛ R2⎞ O = 1.2V • 1 ⎜ + ⎟ R R LTC3873 B Q1 START ⎝ R1⎠ VCC C D1 VCC The external resistor divider is connected to the output 0.1µF GND 8.2V as shown in Figure 5, allowing remote voltage sensing. 3873 F06 Choose resistance values for R1 and R2 to be as large as possible in order to minimize any efficiency loss due to Figure 6 the static current drawn from VOUT, but just small enough so that when VOUT is in regulation, the error caused by Slope Compensation the nonzero input current to the VFB pin is less than 1%. The LTC3873 has built-in internal slope compensation to A good rule of thumb is to choose R1 to be 24k or less. stabilize the control loop against sub-harmonic oscillation. Transformer Design Considerations It also provides the ability to externally increase slope compensation by injecting a ramping current out of its SW Transformer specification and design is perhaps the pin into an external slope compensation resistor (RSL in most critical part of applying the LTC3873 successfully. Figure 5). This current ramp starts at zero right after the In addition to the usual list of caveats dealing with high NGATE pin has been set high. The current rises linearly frequency power transformer design, the following should towards a peak of 20µA at the maximum duty cycle of prove useful. 80%, shutting off once the NGATE pin goes low. A series resistor (RSL) connecting the SW pin to the current sense Turns Ratios resistor (RSENSE) thus develops a ramping voltage drop. Due to the use of the external feedback resistor divider From the perspective of the SW pin, this ramping voltage ratio to set output voltage, the user has relative freedom adds to the voltage across the sense resistor, effectively in selecting a transformer turns ratio to suit a given ap- reducing the current comparator threshold in proportion plication. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, to duty cycle. The amount of reduction in the current etc. can be employed which yield more freedom in setting comparator threshold (ΔVSENSE) can be calculated using total turns and mutual inductance. Simple integer turns the following equation: ratios also facilitate the use of “off-the-shelf” configurable Duty Cycle – 6% transformers such as the Coiltronics VERSA-PAC series ΔVSENSE = 20µA •RSLOPE in applications with high input-to-output voltage ratios. 80% For example, if a 6-winding VERSA-PAC is used with three Note the external programmable slope compensation is windings in series on the primary and three windings in only needed when the internal slope compensation is not parallel on the secondary, a 3:1 turns ratio will be achieved. sufficient. In most applications R Turns ratio can be chosen on the basis of desired duty SL can be shorted. For the LTC3873, when the R cycle. However, remember that the input supply voltage DS(ON) sensing technique is used, the 3873fb 10 For more information www.linear.com/LTC3873
