Datasheet LTC3700 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | Constant Frequency Step-Down DC/DC Controller with LDO Regulator |
| Pages / Page | 16 / 10 — APPLICATIONS INFORMATION. Inductor Value Calculation. RSENSE Selection … |
| File Format / Size | PDF / 221 Kb |
| Document Language | English |
APPLICATIONS INFORMATION. Inductor Value Calculation. RSENSE Selection for Output Current
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LTC3700
U U W U APPLICATIONS INFORMATION
The basic LTC3700 application circuit is shown in␣ Figure␣ 1. SF External component selection for the buck is driven by the RSENSE = ( ) 10 I ( )( ) 100 load requirement and begins with the selection of L1 and OUT RSENSE (= R1). Next, the power MOSFET, M1 and the output diode D1 are selected followed by C
Inductor Value Calculation
IN (= C1) and COUT (= C2). The operating frequency and inductor selection are inter- related in that higher operating frequencies permit the use
RSENSE Selection for Output Current
of a smaller inductor for the same amount of inductor R ripple current. However, this is at the expense of efficiency SENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- due to an increase in MOSFET gate charge losses. oped across RSENSE, the threshold of the comparator The inductance value also has a direct effect on ripple determines the inductor’s peak current. The output cur- current. The ripple current, IRIPPLE, decreases with higher rent the buck can provide is given by: inductance or frequency and increases with higher VIN or V 0 1 . 2 I OUT. The inductor’s peak-to-peak ripple current is given I RIPPLE by: OUT = − RSENSE 2 V V V V I IN OUT OUT D = − + where I RIPPLE RIPPLE is the inductor peak-to-peak ripple current f L V + V IN D ( ) (see Inductor Value Calculation section). where f is the operating frequency. Accepting larger values A reasonable starting point for setting ripple current is of I I RIPPLE allows the use of low inductances, but results in RIPPLE = (0.4)(IOUT). Rearranging the above equation, it higher output voltage ripple and greater core losses. A becomes: reasonable starting point for setting ripple current is 1 IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE RSENSE = for Duty Cycle < 40% occurs at the maximum input voltage. 10 ( ) I (OUT ) However, for operation that is above 40% duty cycle, slope compensation effect has to be taken into consideration to select the appropriate value to provide the required amount of current. Using Figure 2, the value of RSENSE is: 3700f 10
U U W U APPLICATIONS INFORMATION
The basic LTC3700 application circuit is shown in␣ Figure␣ 1. SF External component selection for the buck is driven by the RSENSE = ( ) 10 I ( )( ) 100 load requirement and begins with the selection of L1 and OUT RSENSE (= R1). Next, the power MOSFET, M1 and the output diode D1 are selected followed by C
Inductor Value Calculation
IN (= C1) and COUT (= C2). The operating frequency and inductor selection are inter- related in that higher operating frequencies permit the use
RSENSE Selection for Output Current
of a smaller inductor for the same amount of inductor R ripple current. However, this is at the expense of efficiency SENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- due to an increase in MOSFET gate charge losses. oped across RSENSE, the threshold of the comparator The inductance value also has a direct effect on ripple determines the inductor’s peak current. The output cur- current. The ripple current, IRIPPLE, decreases with higher rent the buck can provide is given by: inductance or frequency and increases with higher VIN or V 0 1 . 2 I OUT. The inductor’s peak-to-peak ripple current is given I RIPPLE by: OUT = − RSENSE 2 V V V V I IN OUT OUT D = − + where I RIPPLE RIPPLE is the inductor peak-to-peak ripple current f L V + V IN D ( ) (see Inductor Value Calculation section). where f is the operating frequency. Accepting larger values A reasonable starting point for setting ripple current is of I I RIPPLE allows the use of low inductances, but results in RIPPLE = (0.4)(IOUT). Rearranging the above equation, it higher output voltage ripple and greater core losses. A becomes: reasonable starting point for setting ripple current is 1 IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE RSENSE = for Duty Cycle < 40% occurs at the maximum input voltage. 10 ( ) I (OUT ) However, for operation that is above 40% duty cycle, slope compensation effect has to be taken into consideration to select the appropriate value to provide the required amount of current. Using Figure 2, the value of RSENSE is: 3700f 10
