Datasheet

LT3505
11
3505fc
Finite transistor bandwidth limits the speed at which the
power switch can be turned on and off, effectively setting
the minimum on-time of the LT3505. For a given output volt-
age, the minimum on-time determines the maximum input
voltage to remain in continuous mode operation, V
IN(PS)
.
See the “Input Voltage Range” section of the datasheet for
more information on determining V
IN(PS)
. For switching
frequencies below 750kHz, operation above V
IN(PS)
(up
to 40V) is safe provided that the system will tolerate the
pulse-skipping behavior outlined in the “Minimum On
Time” section of the datasheet. At switching frequencies
exceeding 750kHz, edge loss limits operation to input
voltages below V
IN(PS)
.
Finite transition time results in a small amount of power
dissipation each time the power switch turns on and off
(edge loss). Edge loss increases with frequency, switch
current, and input voltage.
Input Voltage Frequency Foldback
In constant frequency operation (below V
IN(PS)
) edge
loss only reduces the application effi ciency. However, at
high switching frequencies exceeding 750kHz and input
voltages exceeding V
IN(PS)
, the part operates in pulse-skip-
ping mode and the switch current can increase above the
current limit of the part, 1.75A. This further increases the
power dissipated during switch transitions and increases
die temperature. To remedy the situation a single resis-
tor (R5) and a zener diode (D3) can be added to a typical
LT3505 circuit as shown in Figure 4.
When the input voltage is below 16V, the zener diode
path conducts no current and the current owing out
of the R
T
pin (and through R4) is nominally 0.5V/20k =
25µA, which programs a 2.2MHz switching frequency.
As the input voltage is increased above 16V, the zener
diode begins to conduct and gradually reduces the cur-
rent owing out of the R
T
pin. This mechanism reduces
the switching frequency as the input voltage is increased
above 16V (up to 36V) to ensure that the part constantly
operates in continuous mode without skipping pulses,
thereby preventing the excessive die temperature rise
encountered in pulse-skipping mode.
Although the circuit can be operated indefi nitely above
V
ZENER
, this frequency foldback method is intended to
protect circuits during temporary periods of high input
voltage. For example, in many automotive systems, the
normal operating input range might be 9V to 16V, and
the LT3505 can be programmed to operate above the
AM band (>1.8MHz). At the same time, the circuit must
be able to withstand higher input voltages due to load
dump or double-battery jump starts. During these brief
periods, it is usually acceptable to switch at a frequency
within the AM band.
If the output is shorted while the input voltage is greater than
V
ZENER
, the switching frequency will be reduced to 30kHz
and the part will not be able to recover from the short until
the input voltage is reduced below V
ZENER
(see the following
discussion).
APPLICATIONS INFORMATION
Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit
V
IN
6.7V TO 36V
ON OFF
C3
0.1µF
L1
6.8µH
D2
1N4148
R1
61.9k
C1
10µF
3505 F04
C5
22pF
C4
22pF
C2
1µF
R4
20.0k
R5
806k
R2
11.5k
V
OUT
5V
R3
100k
V
IN
BOOST
GND
FB
V
C
SHDN
SW
LT3505
R
T
D3
BZT52C16T
D1
MBRM140
16V
Input Voltage [V]
0
0
Frequency [MHz] / Load Current [A]
0.50
0.75
1.00
2.25
1.50
10
20
25
0.25
1.75
2.00
2.50
1.25
5 15
30
35
40
LTC3505 • F04b
Switching
Frequency
Maximum
Load Current
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