LT3505IMS8E-TRPBF データシートの表示（PDF） - Linear Technology
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LT3505IMS8E-TRPBF Datasheet PDF : 24 Pages
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LT3505
APPLICATIONS INFORMATION
VSW
20V/DIV
IL
0.5A/DIV
VOUT
200mV/DIV
AC COUPLED
COUT = 10 F
VOUT = 3V
VIN = 30V
2 s/DIV
ILOAD = 0.75A
L = 10 H
RT = 75.0k
Figure 1
3505 F01
VSW
20V/DIV
IL
0.5A/DIV
VOUT
200mV/DIV
AC COUPLED
COUT = 10 F
VOUT = 3V
VIN = 35V
2 s/DIV
ILOAD = 0.75A
L = 10 H
RT = 75.0k
Figure 2
3505 F02
VSW
20V/DIV
IL
0.5A/DIV
VOUT
200mV/DIV
AC COUPLED
COUT = 10µF
VOUT = 3V
VIN = 40V
2µs/DIV
ILOAD = 0.75A
L = 10µH
RT = 75.0k
Figure 3
10
3505 F03
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.
For switching frequencies above 750kHz, the input voltage
must not exceed VIN(PS). See the “Input Voltage Frequency
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.
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
enough to survive prolonged operation under these condi-
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.
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
frequency is programmed by tying a 1% resistor from the RT
pin to ground. Table 1 can be used to select the value of RT.
Minimum on-time and edge loss must be taken into consid-
eration when selecting the intended frequency of operation.
Higher switching frequency increases power dissipation
and lowers efficiency.
3505fc
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