Datasheet LT3999 (Analog Devices) - 8
| Manufacturer | Analog Devices |
| Description | Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control |
| Pages / Page | 16 / 8 — APPLICATIONS INFORMATION Switching Frequency. Oscillator Sync. Duty … |
| File Format / Size | PDF / 310 Kb |
| Document Language | English |
APPLICATIONS INFORMATION Switching Frequency. Oscillator Sync. Duty Cycle. Table 1. Recommended 1% Standard Values. fSW
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LT3999
APPLICATIONS INFORMATION Switching Frequency Oscillator Sync
The LT3999 drives two output power switches out of In applications where a more precise frequency is desired phase, thus the oscillator frequency is two times the actual to accurately place high frequency harmonics, the LT3999 switching frequency of each power switch. The choice oscillator can be synchronized to an external clock. Set of switching frequency is a trade-off between power ef- the internal oscillator frequency 10% to 50% lower than ficiency and the size of capacitive and inductive storage the external sync frequency. The switching frequency is components. one-half the sync frequency. Operating at low switching frequency reduces the switch- Drive the SYNC pin with a 2V or greater square wave. ing losses (transient losses) and consequently improves the The rising edge of the sync square wave will initiate clock power converter efficiency. However, the lower switching discharge. If unused, connect the SYNC pin to ground. frequency requires greater inductance for a given amount of ripple current, resulting in a larger design footprint and
Duty Cycle
higher cost. To run the LT3999 at full duty cycle leave the RDC pin The LT3999 switching frequency is set in the range of 50kHz unconnected. to 1MHz. The value of RT for a given operating frequency Variations in V is chosen from Table 1 or from the following equation: IN are, to a first order, compensated with the LT3999 duty cycle control function. The duty cycle
Table 1. Recommended 1% Standard Values
function is implemented with a resistor divider on VIN
R
connected to the OVLO/DC pin and a resistor to ground
T fSW
316kΩ 50kHz on the RDC pin. Use the following formula to calculate 158kΩ 100kHz the RDC resistor or duty cycle: 76.8kΩ 200kHz 49.9kΩ 300kHz Duty Cycle D ( C)= 1.25•RDC 36.5kΩ 400kHz VIN • RB •RT •4 28kΩ 500kHz RA +RB 22.6kΩ 600kHz 19.1kΩ 700kHz VIN • RB •RT •DC•4 16.2kΩ 800kHz RDC = RA +RB 14kΩ 900kHz 1.25 12.1kΩ 1000kHz where RA and RB are the resistors from the VIN to OVLO/ DC resistor divider and RT is the frequency setting resis- RT k ( Ω)= 1 – 70ns•3.25•1010 tor. See Figure 1. Setting the OVLO/DC pin to be 0.612V 2• fSW at the nominal VIN voltage yields good line regulation over a wide input range. The duty cycle refers to the duty cycle of the individual switch. Normally each switch operates at close to 50% duty cycle. 3999fa 8 For more information www.linear.com/LT3999 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts
APPLICATIONS INFORMATION Switching Frequency Oscillator Sync
The LT3999 drives two output power switches out of In applications where a more precise frequency is desired phase, thus the oscillator frequency is two times the actual to accurately place high frequency harmonics, the LT3999 switching frequency of each power switch. The choice oscillator can be synchronized to an external clock. Set of switching frequency is a trade-off between power ef- the internal oscillator frequency 10% to 50% lower than ficiency and the size of capacitive and inductive storage the external sync frequency. The switching frequency is components. one-half the sync frequency. Operating at low switching frequency reduces the switch- Drive the SYNC pin with a 2V or greater square wave. ing losses (transient losses) and consequently improves the The rising edge of the sync square wave will initiate clock power converter efficiency. However, the lower switching discharge. If unused, connect the SYNC pin to ground. frequency requires greater inductance for a given amount of ripple current, resulting in a larger design footprint and
Duty Cycle
higher cost. To run the LT3999 at full duty cycle leave the RDC pin The LT3999 switching frequency is set in the range of 50kHz unconnected. to 1MHz. The value of RT for a given operating frequency Variations in V is chosen from Table 1 or from the following equation: IN are, to a first order, compensated with the LT3999 duty cycle control function. The duty cycle
Table 1. Recommended 1% Standard Values
function is implemented with a resistor divider on VIN
R
connected to the OVLO/DC pin and a resistor to ground
T fSW
316kΩ 50kHz on the RDC pin. Use the following formula to calculate 158kΩ 100kHz the RDC resistor or duty cycle: 76.8kΩ 200kHz 49.9kΩ 300kHz Duty Cycle D ( C)= 1.25•RDC 36.5kΩ 400kHz VIN • RB •RT •4 28kΩ 500kHz RA +RB 22.6kΩ 600kHz 19.1kΩ 700kHz VIN • RB •RT •DC•4 16.2kΩ 800kHz RDC = RA +RB 14kΩ 900kHz 1.25 12.1kΩ 1000kHz where RA and RB are the resistors from the VIN to OVLO/ DC resistor divider and RT is the frequency setting resis- RT k ( Ω)= 1 – 70ns•3.25•1010 tor. See Figure 1. Setting the OVLO/DC pin to be 0.612V 2• fSW at the nominal VIN voltage yields good line regulation over a wide input range. The duty cycle refers to the duty cycle of the individual switch. Normally each switch operates at close to 50% duty cycle. 3999fa 8 For more information www.linear.com/LT3999 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Package Description Typical Application Related Parts
