LT3695 Series
12
3695fa
APPLICATIONS INFORMATION
FB Resistor Network (LT3695)
The output voltage of the LT3695 is programmed with a
resistor divider between the output and the FB pin. Choose
the resistor values according to:
R
R
V
V
OUT
1
2
0 8
1
=
−
⎛
⎝⎜
⎞
⎠⎟
.
Reference designators refer to the Block Diagram of the
LT3695. 1% resistors are recommended to maintain output
voltage accuracy.
Setting the Switching Frequency
The LT3695 regulators use a constant-frequency PWM
architecture that can be programmed to switch from
250kHz to 2.2MHz by using a resistor tied from the RT
pin to ground. A table showing the necessary R
T
value for
a desired switching frequency is in Table 1.
Table 1. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz)
R
T
VALUE (kΩ)
0.25
158
0.3
127
0.4
90.9
0.5
71.5
0.6
57.6
0.7
47.5
0.8
40.2
0.9
34
1.0
29.4
1.2
22.6
1.4
18.2
1.6
14.7
1.8
12.1
2.0
9.76
2.2
8.06
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
effi ciency, component size, minimum dropout voltage and
maximum input voltage. The advantage of high frequency
operation is that smaller inductor and capacitor values may
be used. The disadvantages are lower effi ciency, lower
maximum input voltage and higher dropout voltage. The
highest acceptable switching frequency (f
SW(MAX)
) for a
given application can be calculated as follows:
f
V
V
t
V
V
V
SW MAX
OUT
D
ON MIN
IN
SW
D
(
)
(
)
(
)
=
+
−
+
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
D
is the catch diode drop (~0.5V) and V
SW
is the
internal switch drop (~0.5V at max load). This equation
shows that lower switching frequency is necessary to
safely accommodate high V
IN
/V
OUT
ratio. Also, as shown
in the Input Voltage Range section, lower frequency allows
a lower dropout voltage. Input voltage range depends on
the switching frequency because the LT3695 regulators’
switch has fi nite minimum on and off times. An internal
timer forces the switch to be off for at least t
OFF(MIN)
per
cycle; this timer has a maximum value of 210ns (250ns
for T
J
> 125°C). On the other hand, delays associated with
turning off the power switch dictate the minimum on-time,
t
ON(MIN)
, before the switch can be turned off; t
ON(MIN)
has a
maximum value of 150ns over temperature. The minimum
and maximum duty cycles that can be achieved taking
minimum on and off times into account are:
DC
MIN
= f
SW
t
ON(MIN)
DC
MAX
= 1 – f
SW
t
OFF(MIN)
where f
SW
is the switching frequency, t
ON(MIN)
is the
minimum switch on time (150ns), and t
OFF(MIN)
is the
minimum switch off time (210ns, 250ns for T
J
> 125°C).
These equations show that the duty cycle range increases
when the switching frequency is decreased.
