LT3685 LINER [Linear Technology], LT3685 Datasheet - Page 12

• Manufacturer Part Number: LT3685
• Description: 36V, 4A, 1.5MHz Synchronous Step-Down Switching Regulator
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
LT3690
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:
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
frequency.
Table 1. Switching Frequency vs R
12
R1 = R2
SWITCHING FREQUENCY (MHz)
⎛
⎜
⎝
V
0.8V
OUT
0.15
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
− 1
⎞
⎟
⎠
T
value for a desired switching
T
Value
R
T
VALUE (kΩ)
72.9
52.2
40.2
32.4
26.8
22.7
19.6
17.0
15.0
13.3
11.8
10.6
9.59
164
117
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
given application can be calculated as follows:
where V
voltage, V
and V
load). This equation shows that slower switching frequency
is necessary to accommodate high V
shown in the Input Voltage Range section, lower frequency
allows a lower dropout voltage. Input voltage range depends
on the switching frequency because the LT3690 switch has
finite minimum on and off times. An internal timer forces
the switch to be off for at least t
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
switch can be turned off; t
of 210ns over temperature. The minimum and maximum
duty cycles that can be achieved taking minimum on and
off times into account are:
where ƒ
minimum switch on-time (210ns), and the t
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 ad-
equate input voltage range (see Input Voltage Range sec-
tion) and keep the inductor and capacitor values small.
DC
DC
ƒ
SW(MAX)
MIN
MAX
SW
SW
IN
is the internal switch drop (0.37V at maximum
LS
= ƒ
= 1 – ƒ
is the typical input voltage, V
is the switching frequency, the t
is the LS switch drop (0.12V at maximum load)
SW
=
t
• t
ON(MIN)
SW
ON(MIN)
• t
OFF(MIN)
V
• V
OUT
(
ON(MIN)
IN
OFF(MIN)
+ V
− V
LS
SW
has a maximum value
IN
/V
per cycle; this timer
+ V
ON(MIN)
OUT
OUT
LS
SW(MAX)
OFF(MIN)
ON(MIN)
)
ratio. Also, as
is the output
before the
) for a
is the
is the
3690f