Datasheet LTC1779 (Analog Devices) - 7
| Manufacturer | Analog Devices |
| Description | 250mA Current Mode Step-Down DC/DC Converter in SOT-23 |
| Pages / Page | 14 / 7 — operaTion (Refer to Functional Diagram). Figure 3. M vs Supply Voltage. … |
| File Format / Size | PDF / 267 Kb |
| Document Language | English |
operaTion (Refer to Functional Diagram). Figure 3. M vs Supply Voltage. applicaTions inForMaTion. Inductor Value Calculation
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LTC1779
operaTion (Refer to Functional Diagram)
The variable M is the ratio of the total switch current to that 60 portion of the switch current that flows through R RSENSE = 18.2Ω SENSE. 55 M is a function of both RSENSE and ROUT of the internal RSENSE = 14Ω 50 power switch, which in turn, is a strong function of sup- RSENSE = 10Ω ply voltage. For values of M refer to Figure 3. In order to 45 guarantee the desired I 40 RSENSE = 6.2Ω PK over the full range of supply voltage, the minimum value of M, corresponding to the M (mA/mA) 35 RSENSE = 2Ω minimum supply voltage seen in the application, should be 30 RSENSE = 0Ω chosen. Note that the selection of RSENSE, and hence the 25 resulting M, is an iterative process. For most applications, a value of R 20 SENSE between 0Ω and 20Ω will be chosen. 0 1 2 3 4 5 6 7 8 9 10 SUPPLY VOLTAGE (V) 1779 F03
Figure 3. M vs Supply Voltage applicaTions inForMaTion
The basic LTC1779 application circuit is shown in Figure maximum input voltage. Use the following equations to 1. External component selection is driven by the load re- calculate L: quirement and begins with the selection of L1 and RSENSE I (= R1). Next, the output diode D1 is selected followed by RIPPLE = 0.4 •IOUT(MAX) CIN (= C1)and COUT(= C2). V ( )•V L IN(MAX) – VOUT OUT = V
Inductor Value Calculation
IN(MAX) • ƒ •IRIPPLE I The inductance value has a direct effect on ripple cur- I RIPPLE L(MAX) =IOUT(MAX) + rent. The ripple current, I 2 RIPPLE, decreases with higher inductance or frequency and increases with higher VIN. and then choose an appropriate L and recalculate the The inductor’s peak-to-peak ripple current is given by: ripple current. (V I IN – VOUT ) VOUT In Burst Mode operation on the LTC1779, the ripple current RIPPLE = VIN • ƒ •L is normally set such that the inductor current is continu- where f is the operating frequency fixed at 550kHz in the ous during the burst periods. Therefore, the peak-to-peak LTC1779. ripple current must not exceed: M(0.030) A smaller value of L results in higher current ripple and IRIPPLE ≤ (R output voltage ripple as well as greater core losses. Larger SENSE +2Ω) values of L decrease the ripple, but require finding physi- This implies a minimum inductance of: cally larger inductors since maximum DC current rating decreases significantly as inductance increases within ⎛ ⎞ L VIN−VOUT VOUT+VD ⎜ ⎟ inductor product types. Generally, by choosing the desired MIN = ⎛ ⎞⎝ VIN+VD ⎠ ripple current based on the maximum output current, the f M(0.030) ⎜ ⎟ ⎝(RSENSE+2Ω) ⎠ inductor value can be calculated from the previous equa- tion. It is typical to choose the inductor so that the ripple (Use VIN(MAX) = VIN) current is about 40% of the maximum output current at 1779fa For more information www.linear.com/LTC1779 7
operaTion (Refer to Functional Diagram)
The variable M is the ratio of the total switch current to that 60 portion of the switch current that flows through R RSENSE = 18.2Ω SENSE. 55 M is a function of both RSENSE and ROUT of the internal RSENSE = 14Ω 50 power switch, which in turn, is a strong function of sup- RSENSE = 10Ω ply voltage. For values of M refer to Figure 3. In order to 45 guarantee the desired I 40 RSENSE = 6.2Ω PK over the full range of supply voltage, the minimum value of M, corresponding to the M (mA/mA) 35 RSENSE = 2Ω minimum supply voltage seen in the application, should be 30 RSENSE = 0Ω chosen. Note that the selection of RSENSE, and hence the 25 resulting M, is an iterative process. For most applications, a value of R 20 SENSE between 0Ω and 20Ω will be chosen. 0 1 2 3 4 5 6 7 8 9 10 SUPPLY VOLTAGE (V) 1779 F03
Figure 3. M vs Supply Voltage applicaTions inForMaTion
The basic LTC1779 application circuit is shown in Figure maximum input voltage. Use the following equations to 1. External component selection is driven by the load re- calculate L: quirement and begins with the selection of L1 and RSENSE I (= R1). Next, the output diode D1 is selected followed by RIPPLE = 0.4 •IOUT(MAX) CIN (= C1)and COUT(= C2). V ( )•V L IN(MAX) – VOUT OUT = V
Inductor Value Calculation
IN(MAX) • ƒ •IRIPPLE I The inductance value has a direct effect on ripple cur- I RIPPLE L(MAX) =IOUT(MAX) + rent. The ripple current, I 2 RIPPLE, decreases with higher inductance or frequency and increases with higher VIN. and then choose an appropriate L and recalculate the The inductor’s peak-to-peak ripple current is given by: ripple current. (V I IN – VOUT ) VOUT In Burst Mode operation on the LTC1779, the ripple current RIPPLE = VIN • ƒ •L is normally set such that the inductor current is continu- where f is the operating frequency fixed at 550kHz in the ous during the burst periods. Therefore, the peak-to-peak LTC1779. ripple current must not exceed: M(0.030) A smaller value of L results in higher current ripple and IRIPPLE ≤ (R output voltage ripple as well as greater core losses. Larger SENSE +2Ω) values of L decrease the ripple, but require finding physi- This implies a minimum inductance of: cally larger inductors since maximum DC current rating decreases significantly as inductance increases within ⎛ ⎞ L VIN−VOUT VOUT+VD ⎜ ⎟ inductor product types. Generally, by choosing the desired MIN = ⎛ ⎞⎝ VIN+VD ⎠ ripple current based on the maximum output current, the f M(0.030) ⎜ ⎟ ⎝(RSENSE+2Ω) ⎠ inductor value can be calculated from the previous equa- tion. It is typical to choose the inductor so that the ripple (Use VIN(MAX) = VIN) current is about 40% of the maximum output current at 1779fa For more information www.linear.com/LTC1779 7
