Datasheet LTC3417A-1 (Analog Devices) - 9
| Manufacturer | Analog Devices |
| Description | Dual Synchronous 1.5A/1A 4MHz Step-Down DC/DC Regulator with POR |
| Pages / Page | 20 / 9 — OPERATION. Dropout Operation. Low Supply Operation. APPLICATIONS … |
| File Format / Size | PDF / 353 Kb |
| Document Language | English |
OPERATION. Dropout Operation. Low Supply Operation. APPLICATIONS INFORMATION. Operating Frequency. Figure 1. Frequency vs RT
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LTC3417A-1
OPERATION Dropout Operation Low Supply Operation
When the input supply voltage decreases toward the output The LTC3417A-1 incorporates an undervoltage lockout voltage, the duty cycle increases to 100%. In this dropout circuit which shuts down the part when the input voltage condition, the PMOS switch is turned on continuously with drops below about 2.07V to prevent unstable operation. the output voltage being equal to the input voltage minus the voltage drops across the internal P-channel MOSFET and inductor.
APPLICATIONS INFORMATION
160 A general LTC3417A-1 application circuit is shown in 140 Figure 4. External component selection is driven by the 120 load requirement, and begins with the selection of the 100 inductors L1 and L2. Once L1 and L2 are chosen, CIN, (kΩ) 80 C T OUT1 and COUT2 can be selected. R 60
Operating Frequency
40 Selection of the operating frequency is a tradeoff between 20 effi ciency and component size. High frequency operation 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 allows the use of smaller inductor and capacitor values. FREQUENCY (MHz) Operation at lower frequencies improves effi ciency by 3417A-1 F01 reducing internal gate charge losses but requires larger
Figure 1. Frequency vs RT
inductance values and/or capacitance to maintain low output ripple voltage. V f OUT MHz ( ) The operating frequency, f O(MAX) 6.67 O, of the LTC3417A-1 is deter- V IN(MAX) mined by pulling the FREQ pin to VIN for 1.5MHz opera- tion or by connecting an external resistor from FREQ to The minimum frequency is limited by leakage and noise ground. The value of the resistor sets the ramp current coupling due to the large resistance of RT. that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by
Inductor Selection
using the following equation: Although the inductor does not infl uence the operating 1.61• 1011 frequency, the inductor value has a direct effect on ripple RT ≈ (Ω) – 16.586kΩ f current. The inductor ripple current, ΔIL, decreases with O higher inductance and increases with higher VIN or for 0.6MHz ≤ f V O ≤ 4MHz. Alternatively, use Figure 1 to OUT. select the value for RT. V OUT The maximum operating frequency is also constrained IL = VOUT 1– f V by the minimum on-time and duty cycle. This can be O • L IN calculated as: Accepting larger values of ΔIL allows the use of low induc- tances, but results in higher output voltage ripple, greater core losses and lower output current capability. 3417a1fa 9
OPERATION Dropout Operation Low Supply Operation
When the input supply voltage decreases toward the output The LTC3417A-1 incorporates an undervoltage lockout voltage, the duty cycle increases to 100%. In this dropout circuit which shuts down the part when the input voltage condition, the PMOS switch is turned on continuously with drops below about 2.07V to prevent unstable operation. the output voltage being equal to the input voltage minus the voltage drops across the internal P-channel MOSFET and inductor.
APPLICATIONS INFORMATION
160 A general LTC3417A-1 application circuit is shown in 140 Figure 4. External component selection is driven by the 120 load requirement, and begins with the selection of the 100 inductors L1 and L2. Once L1 and L2 are chosen, CIN, (kΩ) 80 C T OUT1 and COUT2 can be selected. R 60
Operating Frequency
40 Selection of the operating frequency is a tradeoff between 20 effi ciency and component size. High frequency operation 0 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 allows the use of smaller inductor and capacitor values. FREQUENCY (MHz) Operation at lower frequencies improves effi ciency by 3417A-1 F01 reducing internal gate charge losses but requires larger
Figure 1. Frequency vs RT
inductance values and/or capacitance to maintain low output ripple voltage. V f OUT MHz ( ) The operating frequency, f O(MAX) 6.67 O, of the LTC3417A-1 is deter- V IN(MAX) mined by pulling the FREQ pin to VIN for 1.5MHz opera- tion or by connecting an external resistor from FREQ to The minimum frequency is limited by leakage and noise ground. The value of the resistor sets the ramp current coupling due to the large resistance of RT. that is used to charge and discharge an internal timing capacitor within the oscillator and can be calculated by
Inductor Selection
using the following equation: Although the inductor does not infl uence the operating 1.61• 1011 frequency, the inductor value has a direct effect on ripple RT ≈ (Ω) – 16.586kΩ f current. The inductor ripple current, ΔIL, decreases with O higher inductance and increases with higher VIN or for 0.6MHz ≤ f V O ≤ 4MHz. Alternatively, use Figure 1 to OUT. select the value for RT. V OUT The maximum operating frequency is also constrained IL = VOUT 1– f V by the minimum on-time and duty cycle. This can be O • L IN calculated as: Accepting larger values of ΔIL allows the use of low induc- tances, but results in higher output voltage ripple, greater core losses and lower output current capability. 3417a1fa 9
