lt3990 Linear Technology Corporation, lt3990 Datasheet - Page 9

• Manufacturer Part Number: lt3990
• Description: 60v, 350ma Step-down Regulator With 2.5?a Quiescent Current And Integrated Diodes
• Manufacturer: Linear Technology Corporation

applicaTions inFormaTion
FB Resistor Network
The output voltage is programmed with a resistor divider
between the output and the FB pin. Choose the 1% resis-
tors according to:
Reference designators refer to the Block Diagram. Note
that choosing larger resistors will decrease the quiescent
current of the application circuit.
Setting the Switching Frequency
The LT3990 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.
Table 1. Switching Frequency vs R
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:
SWITCHING FREQUENCY (MHz)
R
f
SW MAX
1
=
(
R
2

 
)
V
=
1 21
0.2
0.3
0.4
0.5
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
OUT
.
t
ON MIN
(
–
1

 
)
V
(
OUT
V
T
IN
value for a desired switching
+
–
T
V
V
SW
Value
D
+
R
V
T
D
VALUE (kΩ)
)
84.5
69.8
60.4
52.3
45.3
825
536
392
301
249
178
133
105
SW(MAX)
) for a
where V
voltage, V
and V
This equation shows that slower switching frequency is
necessary to accommodate high V
Lower frequency also allows a lower dropout voltage.
The input voltage range depends on the switching fre-
quency because the LT3990 switch has finite minimum
on and off times. The switch can turn off for a minimum
of ~160ns but the minimum on-time is a strong function
of temperature. Use the minimum switch on-time curve
(see Typical Performance Characteristics) to design for
an application’s maximum temperature, while adding
about 30% for part-to-part variation. The minimum and
maximum duty cycles that can be achieved taking these
on and off times into account are:
where f
minimum switch on-time, and the t
switch off-time (~160ns). 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 next section) and keep
the inductor and capacitor values small.
Input Voltage Range
The minimum input voltage is determined by either the
LT3990’s minimum operating voltage of 4.2V or by its
maximum duty cycle (as explained in previous section).
The minimum input voltage due to duty cycle is:
where V
output voltage, V
is the internal switch drop (~0.5V at max load), f
the switching frequency (set by RT), and t
minimum switch off-time (160ns). Note that higher switch-
ing frequency will increase the minimum input voltage.
DC
DC
V
IN MIN
SW
MIN
MAX
(
SW
IN(MIN)
IN
is the internal switch drop (~0.5V at max load).
= f
D
= 1 – f
)
is the typical input voltage, V
is the switching frequency, the t
=
is the integrated catch diode drop (~0.7V),
SW
1
–
is the minimum input voltage, V
• t
SW
f
V
D
SW
ON(MIN)
OUT
is the catch diode drop (~0.7V), V
• t
•
t
OFF(MIN)
+
OFF MIN
V
D
(
)
–
OFF(MIN)
V
IN
D
/V
+
OUT
V
OUT
SW
is the minimum
OFF(MIN)
ratio.
LT3990
ON(MIN)
is the output
OUT
is the
is the
SW
is the

3990p
SW
is