Datasheet LTC1874 (Analog Devices) - 7
| Hersteller | Analog Devices |
| Beschreibung | Dual Constant Frequency Current Mode Step-Down DC/DC Controller |
| Seiten / Seite | 12 / 7 — OPERATIO. Slope Compensation and Inductor’s Peak Current. Figure 2. … |
| Dateiformat / Größe | PDF / 190 Kb |
| Dokumentensprache | Englisch |
OPERATIO. Slope Compensation and Inductor’s Peak Current. Figure 2. Percentage of Maximum Output Current vs Duty Cycle
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LTC1874
U OPERATIO Slope Compensation and Inductor’s Peak Current
110 100 The inductor’s peak current is determined by: 90 80 V – . 0 7 (%) I ITH PK = 70 10 R ( ) SENSE OUT(MAX) 60 /I 50 OUT IRIPPLE = 0.4IPK when the controller is operating below 40% duty cycle. 40 AT 5% DUTY CYCLE SF = I I However, once the duty cycle exceeds 40%, slope com- RIPPLE = 0.2IPK 30 AT 5% DUTY CYCLE pensation begins and effectively reduces the peak inductor 20 VIN = 4.2V current. The amount of reduction is given by the curves in 10 0 10 20 30 40 50 60 70 80 90 100 Figure 2. DUTY CYCLE (%) 1874 F02
Figure 2. Percentage of Maximum Output Current vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC1874 application circuit is shown in select the appropriate value to provide the required amount Figure 1. External component selection for each control- of current. Using Figure 2, the value of RSENSE is: ler is driven by the load requirement and begins with the selection of L1 and RSENSE (= R1). Next, the power SF R = MOSFET (M1) and the output diode (D1) are selected SENSE 10 I ( )( )( ) OUT 100 followed by CIN and COUT (= C1). where SF is the “slope factor.”
RSENSE Selection for Output Current
R
Inductor Value Calculation
SENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- The operating frequency and inductor selection are inter- oped across RSENSE, the threshold of the comparator related in that higher operating frequencies permit the use determines the inductor’s peak current. The output cur- of a smaller inductor for the same amount of inductor rent the controller can provide is given by: ripple current. However, this is at the expense of efficiency due to an increase in MOSFET gate charge losses. 0 1 . V 2 I I RIPPLE OUT = − R The inductance value also has a direct effect on ripple SENSE 2 current. The ripple current, IRIPPLE, decreases with higher where IRIPPLE is the inductor peak-to-peak ripple current inductance or frequency and increases with higher VIN or (see Inductor Value Calculation section). VOUT. The inductor’s peak-to-peak ripple current is given by: A reasonable starting point for setting ripple current is IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it + becomes: V V V V I IN OUT OUT D RIPPLE = − f L V + V ( ) IN D 1 RSENSE = for Duty Cycle < 40% 10 I ( )( ) OUT where f is the operating frequency. Accepting larger values of IRIPPLE allows the use of low inductances, but results in However, for operation that is above 40% duty cycle, slope higher output voltage ripple and greater core losses. A compensation effect has to be taken into consideration to reasonable starting point for setting ripple current is 7
U OPERATIO Slope Compensation and Inductor’s Peak Current
110 100 The inductor’s peak current is determined by: 90 80 V – . 0 7 (%) I ITH PK = 70 10 R ( ) SENSE OUT(MAX) 60 /I 50 OUT IRIPPLE = 0.4IPK when the controller is operating below 40% duty cycle. 40 AT 5% DUTY CYCLE SF = I I However, once the duty cycle exceeds 40%, slope com- RIPPLE = 0.2IPK 30 AT 5% DUTY CYCLE pensation begins and effectively reduces the peak inductor 20 VIN = 4.2V current. The amount of reduction is given by the curves in 10 0 10 20 30 40 50 60 70 80 90 100 Figure 2. DUTY CYCLE (%) 1874 F02
Figure 2. Percentage of Maximum Output Current vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC1874 application circuit is shown in select the appropriate value to provide the required amount Figure 1. External component selection for each control- of current. Using Figure 2, the value of RSENSE is: ler is driven by the load requirement and begins with the selection of L1 and RSENSE (= R1). Next, the power SF R = MOSFET (M1) and the output diode (D1) are selected SENSE 10 I ( )( )( ) OUT 100 followed by CIN and COUT (= C1). where SF is the “slope factor.”
RSENSE Selection for Output Current
R
Inductor Value Calculation
SENSE is chosen based on the required output current. With the current comparator monitoring the voltage devel- The operating frequency and inductor selection are inter- oped across RSENSE, the threshold of the comparator related in that higher operating frequencies permit the use determines the inductor’s peak current. The output cur- of a smaller inductor for the same amount of inductor rent the controller can provide is given by: ripple current. However, this is at the expense of efficiency due to an increase in MOSFET gate charge losses. 0 1 . V 2 I I RIPPLE OUT = − R The inductance value also has a direct effect on ripple SENSE 2 current. The ripple current, IRIPPLE, decreases with higher where IRIPPLE is the inductor peak-to-peak ripple current inductance or frequency and increases with higher VIN or (see Inductor Value Calculation section). VOUT. The inductor’s peak-to-peak ripple current is given by: A reasonable starting point for setting ripple current is IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it + becomes: V V V V I IN OUT OUT D RIPPLE = − f L V + V ( ) IN D 1 RSENSE = for Duty Cycle < 40% 10 I ( )( ) OUT where f is the operating frequency. Accepting larger values of IRIPPLE allows the use of low inductances, but results in However, for operation that is above 40% duty cycle, slope higher output voltage ripple and greater core losses. A compensation effect has to be taken into consideration to reasonable starting point for setting ripple current is 7