LT3505
APPLICATIONS INFORMATION
Now the required on time has decreased below the
minimum on time of 130ns. Instead of the switch pulse
width becoming narrower to accommodate the lower duty
cycle requirement, the switch pulse width remains fixed
at 130ns. In Figure 2 the inductor current ramps up to a
value exceeding the load current and the output ripple
increases to ~200mV. The part then remains off until the
output voltage dips below 100% of the programmed value
before it begins switching again.
V
SW
20V/DIV
I
L
0.5A/DIV
V
OUT
200mV/DIV
AC COUPLED
3505 F01
2 s/DIV
= 0.75A
Forswitchingfrequenciesabove750kHz,theinputvoltage
mustnotexceedVIN(PS).Seethe“InputVoltageFrequency
Foldback” section of the datasheet for a circuit solution
that provides safe operation above VIN(PS) at switching
frequencies exceeding 750kHz. For switching frequencies
below 750kHz, operation above VIN(PS) is safe and will
not damage the part as long as the output voltage stays
in regulation and the inductor does not saturate. Figure
3 shows the switching waveforms of a 750kHz applica-
tion when the input voltage is increased to its absolute
maximum rating of 40V.
C
V
V
= 10 F
= 3V
I
LOAD
L = 10 H
R = 75.0k
T
OUT
OUT
IN
= 30V
Figure 1
V
SW
20V/DIV
I
L
0.5A/DIV
As the input voltage increases, the inductor current ramp
rate increases, the number of skipped pulses increases
and the output voltage ripple increases. The part is robust
enoughtosurviveprolongedoperationunderthesecondi-
tions as long as the programmed switching frequency is
less than 750kHz and the peak inductor current does not
exceed 2.2A. Inductor current saturation may further limit
performance in this operating regime.
V
OUT
200mV/DIV
AC COUPLED
3505 F02
2 s/DIV
= 0.75A
C
V
V
= 10 F
= 3V
I
LOAD
OUT
OUT
IN
L = 10 H
= 35V
R
= 75.0k
T
Figure 2
Frequency Selection
The maximum frequency that the LT3505 can be pro-
grammed to is 3MHz. The minimum frequency that the
LT3505 can be programmed to is 200kHz. The switching
frequencyisprogrammedbytyinga1%resistorfromtheRT
pintoground.Table1canbeusedtoselectthevalueofRT.
Minimumon-timeandedgelossmustbetakenintoconsid-
erationwhenselectingtheintendedfrequencyofoperation.
Higher switching frequency increases power dissipation
andlowersefficiency.
V
SW
20V/DIV
I
L
0.5A/DIV
V
OUT
200mV/DIV
AC COUPLED
3505 F03
2µs/DIV
C
V
V
= 10µF
I
= 0.75A
OUT
OUT
IN
LOAD
= 3V
L = 10µH
= 40V
R
= 75.0k
T
Figure 3
3505fc
10
Linear [ Linear ]
