Datasheet LTC3873-5 (Analog Devices) - 9

LTC3873-5
APPLICATIONS INFORMATION VCC Bias Power
resistor (RSL) connecting the SW pin to the current sense resistor (R The V SENSE) thus develops a ramping voltage drop. CC pin must be bypassed to the GND pin with a From the perspective of the SW pin, this ramping voltage minimum 10μF ceramic or tantalum capacitor located adds to the voltage across the sense resistor, effectively immediately adjacent to the two pins. Proper supply by- reducing the current comparator threshold in proportion passing is necessary to supply the high transient currents to duty cycle. The amount of reduction in the current required by the MOSFET gate driver. comparator threshold (ΔVSENSE) can be calculated using For maximum fl exibility, the LTC3873-5 is designed so the following equation: that it can be operated from voltages well beyond the LTC3873-5’s absolute maximum ratings. In the simplest Δ Duty Cycle 6 V = – % 20 A μ •R case, the LTC3873-5 can be powered with a resistor con- SENSE SLOPE 8 % 0 nected between the input voltage and VCC. The built-in shunt regulator limits the voltage on the V Note the external programmable slope compensation is CC pin to around 9.3V as long as the shunt regulator is not forced to sink more only needed when the internal slope compensation is not than 25mA. This powering scheme has the drawback that suffi cient. In some applications RSL can be shorted. For the power loss in the resistor reduces converter effi ciency the LTC3873-5, when the RDS(ON) sensing technique is and the 25mA shunt regulator maximum may limit the used, the ringing on the SW pin disrupts the tiny slope maximum-minimum range of input voltage. compensation current out of the pin. It is not recommended to add external slope compensation in this case. The circuit in Figure 5 shows a second way to power the LTC3873-5. An external series pre-regulator consisting of
Output Voltage Programming
series pass transistor Q1, zener diode D1 and bias resis- tor R The output voltage is set by a resistor divider according B brings VCC to at least 7.6V nominal, well above the undervoltage lockout threshold. to the following formula: V ⎛ R ⎞ 2 IN V 1 2 . V O = • 1+ ⎝⎜ ⎠⎟ R1 R LTC3873-5 B Q1 VCC The external resistor divider is connected to the output C D1 VCC GND as shown in Figure 4, allowing remote voltage sensing. 0.1μF 8.2V Choose resistance values for R1 and R2 to be as large as 38735 F05 possible in order to minimize any effi ciency loss due to
Figure 5. External Pre-Regulator for V
the static current drawn from V
CC Bias Power
OUT, 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%. A good rule of thumb is to choose R1 to be 24k or less. The LTC3873-5 has built-in internal slope compensation to stabilize the control loop against sub-harmonic oscilla-
Transformer Design Considerations
tion. It also provides the ability to externally increase slope compensation by injecting a ramping current out of its Transformer specifi cation and design is perhaps the most SW pin into an external slope compensation resistor (R critical part of applying the LTC3873-5 successfully. In SL in Figure 2). This current ramp starts at zero right after addition to the usual list of caveats dealing with high fre- the NGATE pin has been high. The current rises linearly quency 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 38735fb 9