Datasheet

16
LTC3736-1
37361f
Operating Frequency
When spread spectrum operation is enabled (SSDIS =
GND), the frequency of the LTC3736-1 is randomly varied
over the range of frequencies between 450kHz and 580kHz.
In this case, a capacitor (1nF to 4.7nF) should be connected
between the FREQ pin and SGND (or V
IN
) to smooth out the
changes in frequency. This not only provides a smoother
frequency spectrum but also ensures that the switching
regulator remains stable by preventing abrupt changes
in frequency. A value of 2200pF is suitable in most
applications.
When the spread spectrum operation is disabled (SSDIS =
V
IN
), the LTC3736-1’s frequency may be selected from
among three discrete, constant frequencies using the FREQ
pin. Floating the FREQ pin selects 550kHz operation; tying
this pin to V
IN
selects 750kHz, while tying this pin to GND
selects 300kHz. Table 2 summarizes the different states in
which the FREQ pin can be used.
Table 2
FREQ PIN SSDIS PIN FREQUENCY
0V V
IN
300kHz
Floating V
IN
550kHz
V
IN
V
IN
750kHz
Capacitor to GND GND Spread Spectrum (450kHz to 580kHz)
or V
IN
Note that when spread spectrum operation is disabled, the
LTC3736-1 operates like the standard, constant frequency
LTC3736, except that at light loads, the LTC3736-1 oper-
ates in pulse skipping mode. This mode is not available on
the LTC3736 unless the device is synchronized to an ex-
ternal clock signal using its phase-locked loop (PLL). Thus,
if an LTC3736 with pulse skipping function is needed, then
the LTC3736-1 with spread spectrum disabled is the appro-
priate solution. Table 3 summarizes the key differences in
the available features on the LTC3736 and LTC3736-1.
Table 3
AVAILABLE FEATURES/OPTIONS LTC3736 LTC3736-1
Selectable Constant Frequency Yes Yes
Spread Spectrum No Yes
Synchronizable (PLL) Yes No
Burst Mode
®
Yes No
Forced Continuous Mode Yes No
Pulse Skipping Mode When Synchronized Yes
Inductor Value Calculation
Given the desired input and output voltages, the inductor
value and operating frequency f
OSC
directly determine the
inductor’s peak-to-peak ripple current:
I
V
V
VV
fL
RIPPLE
OUT
IN
IN OUT
OSC
=
⎛
⎝
⎜
⎞
⎠
⎟
–
•
Lower ripple current reduces core losses in the inductor,
ESR losses in the output capacitors, and output voltage
ripple. Thus, highest efficiency operation is obtained at
low frequency with a small ripple current. Achieving this,
however, requires a large inductor.
A reasonable starting point is to choose a ripple current
that is about 40% of I
OUT(MAX)
. Note that the largest ripple
current occurs at the highest input voltage. To guarantee
that ripple current does not exceed a specified maximum,
the inductor should be chosen according to:
L
VV
fI
V
V
IN OUT
OSC RIPPLE
OUT
IN
≥
–
•
•
Inductor Core Selection
Once the inductance value is determined, the type of
inductor must be selected. High efficiency converters
generally cannot afford the core loss found in low cost
powdered iron cores, forcing the use of ferrite, molyper-
malloy or Kool Mµ
®
cores. Actual core loss is independent
of core size for a fixed inductor value, but it is very
dependent on inductance selected. As inductance in-
creases, core losses go down. Unfortunately, increased
inductance requires more turns of wire and therefore
copper losses will increase.
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can
concentrate on copper loss and preventing saturation.
Ferrite core material saturates “hard,” which means that
inductance collapses abruptly when the peak design cur-
rent is exceeded. This results in an abrupt increase in
inductor ripple current and consequent output voltage
ripple. Do not allow the core to saturate!
APPLICATIO S I FOR ATIO
WUUU
Burst Mode is a registered trademark of Linear Technology Corporation.
Kool Mµ is a registered trademark of Magnetics, Inc.