LT3970IMS-5#TRPBF Linear Technology, LT3970IMS-5#TRPBF Datasheet - Page 10

• Manufacturer Part Number: LT3970IMS-5#TRPBF
• Description: IC BUCK 5V 0.35A 10MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3970IMS-5#TRPBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 5V
• Current - Output: 350mA
• Frequency - Switching: 200kHz ~ 2.25MHz
• Voltage - Input: 4.2 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

LT3970/LT3970-3.3/LT3970-5
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 LT3970 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
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
given application can be calculated as follows:
10
SWITCHING FREQUENCY (MHz)
R1=R2
f
SW MAX
(
)
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)
)
80.6
68.1
57.6
49.9
42.2
768
499
357
280
226
158
124
100
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 frequency
because the LT3970 switch has fi nite minimum on and off
times. The switch can turn on for a minimum of ~150ns and
turn off for a minimum of ~160ns (note that the minimum
on-time is a strong function of temperature). This means
that the minimum and maximum duty cycles are:
where f
minimum switch on-time (~150ns), and the t
the minimum 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
LT3970’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
the output voltage, V
V
is the switching frequency (set by RT), and t
the minimum switch off-time (160ns). Note that higher
switching frequency will increase the minimum input
voltage. If a lower dropout voltage is desired, a lower
switching frequency should be used.
SW
DC
DC
V
IN MIN
is the internal switch drop (~0.5V at max load), f
SW
MIN
MAX
(
SW
IN
IN(MIN)
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
–
• t
is the minimum input voltage, V
SW
f
V
SW
ON(MIN)
OUT
• t
•
D
t
OFF(MIN)
+
OFF MIN
is the catch diode drop (~0.7V),
V
D
(
)
–
V
IN
D
/V
+
OUT
V
OUT
SW
ratio.
ON(MIN)
is the output
OFF(MIN)
OFF(MIN)
OUT
is the
3970fa
SW
is
is
is