LT3690 LINER [Linear Technology], LT3690 Datasheet - Page 12

• Manufacturer Part Number: LT3690
• Description: 42V, 2.5A Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LT8610
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:
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
If low input quiescent current and good light-load efficiency
are desired, use large resistor values for the FB resistor
divider. The current flowing in the divider acts as a load
current, and will increase the no-load input current to the
converter, which is approximately:
where 1.7μA is the quiescent current of the LT8610 and
the second term is the current in the feedback divider
reflected to the input of the buck operating at its light
load efficiency n. For a 3.3V application with R1 = 1M and
R2 = 412k, the feedback divider draws 2.3μA. With V
12V and n = 80%, this adds 0.8μA to the 1.7μA quiescent
current resulting in 2.5μA no-load current from the 12V
supply. Note that this equation implies that the no-load
current is a function of V
Performance Characteristics section.
When using large FB resistors, a 4.7pF to 10pF phase-lead
capacitor should be connected from V
Setting the Switching Frequency
The LT8610 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2.2MHz
by using a resistor tied from the RT pin to ground. A table
showing the necessary R
frequency is in Table 1.
The R
can be calculated using:
12
R1= R2
I
R
Q
T
= 1.7µA +
T
=
resistor required for a desired switching frequency
46.5
f
SW
⎛
⎝ ⎜
0.970V
V
– 5.2
OUT
⎛
⎝ ⎜
R1+R2
V
OUT
– 1
⎞
⎠ ⎟
⎞
⎠ ⎟
IN
T
⎛
⎝ ⎜
; this is plotted in the Typical
value for a desired switching
V
V
OUT
IN
⎞
⎠ ⎟
⎛
⎝ ⎜
n
1
⎞
⎠ ⎟
OUT
to FB.
IN
(1)
(2)
(3)
=
where R
quency in MHz.
Table 1. SW Frequency vs R
Operating Frequency Selection and Trade-Offs
Selection of the operating frequency is a trade-off between
efficiency, component size, and input voltage range. The
advantage of high frequency operation is that smaller induc-
tor and capacitor values may be used. The disadvantages
are lower efficiency and a smaller input voltage range.
The highest switching frequency (f
application can be calculated as follows:
where V
voltage, V
drops (~0.3V, ~0.15V, respectively at maximum load)
and t
Electrical Characteristics). This equation shows that a
slower switching frequency is necessary to accommodate
a high V
For transient operation, V
lute maximum rating of 42V regardless of the R
however the LT8610 will reduce switching frequency as
necessary to maintain control of inductor current to as-
sure safe operation.
f
SW(MAX)
ON(MIN)
IN
IN
T
/V
is in kΩ and f
f
SW(TOP)
SW
is the typical input voltage, V
OUT
=
0.2
0.3
0.4
0.5
0.6
0.7
0.8
1.0
1.2
1.6
1.8
2.0
2.2
is the minimum top switch on-time (see the
14
(MHz)
t
ON(MIN)
ratio.
and V
T
Value
(
SW
V
SW(BOT)
V
IN
IN
OUT
is the desired switching fre-
– V
may go as high as the abso-
+ V
SW(TOP)
are the internal switch
SW(BOT)
SW(MAX)
R
+ V
OUT
T
88.7
71.5
60.4
52.3
41.2
33.2
28.0
23.7
20.5
18.2
15.8
232
150
110
(kΩ)
SW(BOT)
) for a given
is the output
T
value,
)
8610p
(4)