Datasheet

LT3690
12
3690fa
applicaTions inForMaTion
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the resistor
values according to:
R1 = R2
V
OUT
0.8V
−1






Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
Setting the Switching Frequency
The LT3690 uses a constant frequency PWM architecture
that can be programmed to switch from 150kHz to 1.5MHz
by using a resistor tied from the RT pin to ground. Table 1
shows the necessary R
T
value for a desired switching
frequency.
Table 1. Switching Frequency vs R
T
Value
SWITCHING FREQUENCY (MHz) R
T
VALUE (kΩ)
0.15 164
0.2 117
0.3 72.9
0.4 52.2
0.5 40.2
0.6 32.4
0.7 26.8
0.8 22.7
0.9 19.6
1.0 17.0
1.1 15.0
1.2 13.3
1.3 11.8
1.4 10.6
1.5 9.59
Operating Frequency Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, 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 efficiency, 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:
ƒ
SW(MAX)
=
V
OUT
+ V
LS
t
ON(MIN)
• V
IN
− V
SW
+ V
LS
( )
where V
IN
is the typical input voltage, V
OUT
is the output
voltage, V
LS
is the LS switch drop (0.12V at maximum
load) and V
SW
is the internal switch drop (0.37V at maxi-
mum load). This equation shows that slower switching
frequency is necessary to 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 be-
cause the LT3690 switch has finite 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 over temperature. 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 210ns (250ns for
T
J
> 125°C) over temperature. The minimum and maxi-
mum duty cycles that can be achieved taking minimum
on and off times into account are:
DC
MIN
= ƒ
SW
• t
ON(MIN)
DC
MAX
= 1 – ƒ
SW
• t
OFF(MIN)
where ƒ
SW
is the switching frequency, the t
ON(MIN)
is the
minimum switch on-time (210ns; 250ns for T
J
> 125°C),
and the t
OFF(MIN)
is the minimum switch off-time (210ns).
These equations show that duty cycle range increases
when switching frequency is decreased.
A good choice of switching frequency should allow
adequate input voltage range (see Input Voltage Range
section) and keep the inductor and capacitor values small.