Datasheet LTC3801, LTC3801B (Analog Devices) - 7

LTC3801/LTC3801B
U OPERATIO (Refer to the Functional Diagram)
inductor current. The amount of reduction is given by the 115 100 curve in Figure 3. 105 90 LTC3801 SLOPE FACTOR (%) LTC3801 95 80
Soft-Start
LTC3801B 85 70 An internal default soft-start circuit is employed at power- 75 up and/or when coming out of shutdown. The soft-start 60 65 circuit works by internally clamping the voltage at the TRIP VOLTAGE (mV) 50 I 55 TH/RUN pin to the corresponding zero current level and 40 gradually raising the clamp voltage such that the minimum 45 VIN = 4.2V TA = 25°C time required for the programmed switch current to reach 35 30 20 30 40 50 60 70 80 90 100 its maximum is approximately 0.6ms. After the soft-start DUTY CYCLE (%) circuit has timed out, it is disabled until the part is put in 3801 F03 shutdown again or the input supply is cycled.
Figure 3. Maximum Current Limit Trip Voltage vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC3801/LTC3801B application circuit is shown However, for operation that is above 40% duty cycle, slope on the front page of this data sheet. External component compensation effect has to be taken into consideration to selection is driven by the load requirement and begins with select the appropriate value to provide the required amount the selection of the inductor and RSENSE. Next, the power of current. Using Figure 3, the value of RSENSE is: MOSFET and the output diode are selected followed by the input bypass capacitor C SF IN and output bypass capacitor RSENSE = COUT. ( ) 10 I ( OUT)( ) 100
R
where SF is the “Slope Factor.”
SENSE Selection for Output Current
RSENSE is chosen based on the required output current.
Inductor Value Calculation
With the current comparator monitoring the voltage developed across R The operating frequency and inductor selection are inter- SENSE, the threshold of the compara- related in that higher operating frequencies permit the use tor determines the inductor’s peak current. The output of a smaller inductor for the same amount of inductor current the LTC3801 can provide is given by: ripple current. However, this is at the expense of efficiency 0.117 I I RIPPLE due to an increase in MOSFET gate charge losses. OUT = − RSENSE 2 The inductance value also has a direct effect on ripple cur- where IRIPPLE is the inductor peak-to-peak ripple current rent. The ripple current, IRIPPLE, decreases with higher in- (see Inductor Value Calculation section). For the LTC3801B ductance or frequency and increases with higher VIN or use 104mV in the previous equation and follow through VOUT. The inductor’s peak-to-peak ripple current is given by: the analysis using that number. V V  V V IN OUT OUT D  A reasonable starting point for setting ripple current is IRIPPLE = − + f L  V + V  ( ) I IN D RIPPLE = (0.4)(IOUT). Rearranging the above equation, it becomes: where f is the operating frequency. Accepting larger values 1 of IRIPPLE allows the use of low inductances, but results in RSENSE = for Duty Cycle < 40% higher output voltage ripple and greater core losses. A 10 ( ) I ( OUT) reasonable starting point for setting ripple current is sn3801 3801fs 7