LT3975
APPLICATIONS INFORMATION
light loads, the part will go to sleep between groups of
pulses, so the quiescent current of the part will still be low,
but not as low as in Burst Mode operation. The quiescent
current in a typical application when synchronized with an
external clock is 11µA at no load. Holding the SYNC pin
DC high yields no advantages in terms of output ripple or
minimum load to full frequency, so is not recommended.
Table 1. Switching Frequency vs RT Value
SWITCHING FREQUENCY (MHz)
R VALUE (kΩ)
T
0.2
0.3
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
294
182
130
78.7
54.9
41.2
32.4
26.1
21.5
17.8
14.7
12.4
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:
VOUT
1.197V
R1=R2
–1
Operating Frequency Trade-Offs
Reference designators refer to the Block Diagram. 1%
resistors are recommended to maintain output voltage
accuracy.
Selectionoftheoperatingfrequencyisatrade-offbetween
efficiency,componentsize,minimumdropoutvoltage,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, and
lower maximum input voltage. The highest acceptable
The total resistance of the FB resistor divider should be
selected to be as large as possible to enhance low current
performance. The resistor divider generates a small load
on the output, which should be minimized to optimize the
low supply current at light loads.
switching frequency (f
) for a given application
SW(MAX)
can be calculated as follows:
WhenusinglargeFBresistors,a10pFphaseleadcapacitor
VOUT + VD
should be connected from V
to FB.
fSW(MAX)
=
OUT
tON(MIN) V – VSW + VD
(
)
OUT
IN
Setting the Switching Frequency
where V is the typical input voltage, V
is the output
IN
The LT3975 uses a constant frequency PWM architecture
that can be programmed to switch from 200kHz to 2MHz
by using a resistor tied from the RT pin to ground. A table
voltage, V is the catch diode drop (~0.5V), and V is
D
SW
the internal switch drop (~0.22V at max load). This equa-
tion shows that slower switching frequency is necessary
showing the necessary R value for a desired switching
T
to safely accommodate high V /V
ratio. This is due
IN OUT
frequency is in Table 1.
to the limitation on the LT3975’s minimum on-time. The
minimum on-time is a strong function of temperature.
Use the typical minimum on-time curve to design for an
application’s maximum temperature, while adding about
30%forpart-to-partvariation.Theminimumdutycyclethat
can be achieved taking minimum on time into account is:
To estimate the necessary R value for a desired switching
T
frequency, use the equation:
51.1
RT =
–9.27
1.09
f
(
)
SW
DC
= f • t
SW ON(MIN)
MIN
where R is in kΩ and f is in MHz.
T
SW
where f is the switching frequency, the t
is the
ON(MIN)
SW
minimum switch on-time.
3975f
12
Linear [ Linear ]
