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LT3999 Datasheet(PDF) 8 Page - Linear Technology

Part No. LT3999
Description  Low Noise, 1A, 1MHz Push-Pull DC/DC Driver with Duty Cycle Control
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Maker  LINER [Linear Technology]
Homepage  http://www.linear.com
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LT3999 Datasheet(HTML) 8 Page - Linear Technology

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LT3999
8
3999fa
For more information www.linear.com/LT3999
APPLICATIONS INFORMATION
Switching Frequency
The LT3999 drives two output power switches out of
phase, thus the oscillator frequency is two times the actual
switching frequency of each power switch. The choice
of switching frequency is a trade-off between power ef-
ficiency and the size of capacitive and inductive storage
components.
Operating at low switching frequency reduces the switch-
inglosses(transientlosses)andconsequentlyimprovesthe
power converter efficiency. However, the lower switching
frequency requires greater inductance for a given amount
of ripple current, resulting in a larger design footprint and
higher cost.
TheLT3999switchingfrequencyissetintherangeof50kHz
to 1MHz. The value of RT for a given operating frequency
is chosen from Table 1 or from the following equation:
Table 1. Recommended 1% Standard Values
RT
fSW
316kΩ
50kHz
158kΩ
100kHz
76.8kΩ
200kHz
49.9kΩ
300kHz
36.5kΩ
400kHz
28kΩ
500kHz
22.6kΩ
600kHz
19.1kΩ
700kHz
16.2kΩ
800kHz
14kΩ
900kHz
12.1kΩ
1000kHz
RT kΩ
( )=
1
2• fSW
– 70ns
•3.25•1010
Oscillator Sync
In applications where a more precise frequency is desired
to accurately place high frequency harmonics, the LT3999
oscillator can be synchronized to an external clock. Set
the internal oscillator frequency 10% to 50% lower than
the external sync frequency. The switching frequency is
one-half the sync frequency.
Drive the SYNC pin with a 2V or greater square wave.
The rising edge of the sync square wave will initiate clock
discharge. If unused, connect the SYNC pin to ground.
Duty Cycle
To run the LT3999 at full duty cycle leave the RDC pin
unconnected.
Variations in VIN are, to a first order, compensated with
the LT3999 duty cycle control function. The duty cycle
function is implemented with a resistor divider on VIN
connected to the OVLO/DC pin and a resistor to ground
on the RDC pin. Use the following formula to calculate
the RDC resistor or duty cycle:
Duty Cycle DC
( )=
1.25•RDC
VIN
RB
RA +RB
•RT •4
RDC =
VIN
RB
RA +RB
•RT •DC•4
1.25
where RA and RB are the resistors from the VIN to OVLO/
DC resistor divider and RT is the frequency setting resis-
tor. See Figure 1. Setting the OVLO/DC pin to be 0.612V
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.


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