Datasheet LTC1875 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | 15µA Quiescent Current 1.5A Monolithic Synchronous Step-Down Regulator |
| Pages / Page | 20 / 10 — OPERATIO. Slope Compensation and Inductor Peak Current. Figure 3. Maximum … |
| File Format / Size | PDF / 269 Kb |
| Document Language | English |
OPERATIO. Slope Compensation and Inductor Peak Current. Figure 3. Maximum Inductor Peak Current vs Duty Cycle
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LTC1875
U OPERATIO Slope Compensation and Inductor Peak Current
2200 VIN = 3V Slope compensation is required in order to prevent sub- 2000 harmonic oscillation at high duty cycles. It is accom- 1800 plished by internally adding a compensating ramp to the 1600 inductor current signal at duty cycles in excess of 40%. As a result, the maximum inductor peak current is reduced for 1400 duty cycles > 40%. This is shown in the decrease of the 1200 inductor peak current as a function of duty cycle graph in 1000 Figure 3. MAXIMUM INDUCTOR PEAK CURRENT (mA) 800 0 20 40 60 80 100 DUTY CYCLE (%) 1875 F03
Figure 3. Maximum Inductor Peak Current vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC1875 application circuit is shown on the first A reasonable starting point for setting ripple current is page of this data sheet. External component selection is ∆IL = 0.3(IMAX). driven by the load requirement and begins with the selec- The inductor value also has an effect on Burst Mode tion of L followed by CIN and COUT. operation. The transition to low current operation begins when the inductor current peaks fall to approximately
Inductor Value Calculation
500mA. Lower inductor values (higher ∆IL) will cause this The inductor selection will depend on the operating fre- to occur at lower load currents, which can cause a dip in quency of the LTC1875. The internal nominal frequency is efficiency in the upper range of low current operation. In 550kHz, but can be externally synchronized from 350kHz Burst Mode operation, lower inductance values will cause to 750kHz. the burst frequency to increase. The operating frequency and inductor selection are inter-
Inductor Selection
related in that higher operating frequencies allow the use of smaller inductor and capacitor values. However, oper- The inductor should have a saturation current rating ating at a higher frequency results in lower efficiency greater than the peak inductor current set by the current because of increased switching losses. comparator of LTC1875. Also, consideration should be given to the resistance of the inductor. Inductor conduc- The inductor value has a direct effect on ripple current. The tion losses are directly proportional to the DC resistance ripple current ∆IL decreases with higher inductance or of the inductor. Manufacturers sometimes provide maxi- frequency and increases with higher input voltages. mum current ratings based on the allowable losses in the inductor. ∆ 1 V I OUT L = ( VOUT 1– (1) f)( ) L VIN Suitable inductors are available from Coilcraft, Coiltron- Accepting larger values of ∆I ics, Dale, Sumida, Toko, Murata, Panasonic and other L allows the use of smaller inductors, but results in higher output voltage ripple. manufacturers. 1875f 10
U OPERATIO Slope Compensation and Inductor Peak Current
2200 VIN = 3V Slope compensation is required in order to prevent sub- 2000 harmonic oscillation at high duty cycles. It is accom- 1800 plished by internally adding a compensating ramp to the 1600 inductor current signal at duty cycles in excess of 40%. As a result, the maximum inductor peak current is reduced for 1400 duty cycles > 40%. This is shown in the decrease of the 1200 inductor peak current as a function of duty cycle graph in 1000 Figure 3. MAXIMUM INDUCTOR PEAK CURRENT (mA) 800 0 20 40 60 80 100 DUTY CYCLE (%) 1875 F03
Figure 3. Maximum Inductor Peak Current vs Duty Cycle U U W U APPLICATIO S I FOR ATIO
The basic LTC1875 application circuit is shown on the first A reasonable starting point for setting ripple current is page of this data sheet. External component selection is ∆IL = 0.3(IMAX). driven by the load requirement and begins with the selec- The inductor value also has an effect on Burst Mode tion of L followed by CIN and COUT. operation. The transition to low current operation begins when the inductor current peaks fall to approximately
Inductor Value Calculation
500mA. Lower inductor values (higher ∆IL) will cause this The inductor selection will depend on the operating fre- to occur at lower load currents, which can cause a dip in quency of the LTC1875. The internal nominal frequency is efficiency in the upper range of low current operation. In 550kHz, but can be externally synchronized from 350kHz Burst Mode operation, lower inductance values will cause to 750kHz. the burst frequency to increase. The operating frequency and inductor selection are inter-
Inductor Selection
related in that higher operating frequencies allow the use of smaller inductor and capacitor values. However, oper- The inductor should have a saturation current rating ating at a higher frequency results in lower efficiency greater than the peak inductor current set by the current because of increased switching losses. comparator of LTC1875. Also, consideration should be given to the resistance of the inductor. Inductor conduc- The inductor value has a direct effect on ripple current. The tion losses are directly proportional to the DC resistance ripple current ∆IL decreases with higher inductance or of the inductor. Manufacturers sometimes provide maxi- frequency and increases with higher input voltages. mum current ratings based on the allowable losses in the inductor. ∆ 1 V I OUT L = ( VOUT 1– (1) f)( ) L VIN Suitable inductors are available from Coilcraft, Coiltron- Accepting larger values of ∆I ics, Dale, Sumida, Toko, Murata, Panasonic and other L allows the use of smaller inductors, but results in higher output voltage ripple. manufacturers. 1875f 10
