Datasheet

LTC3458L
12
3458lfa
Input Capacitor Selection
The input filter capacitor reduces peak currents drawn
from the input source and reduces input switching noise.
In most applications >1μF per amp of peak input current
is recommended. See Table 3 for a list of capacitor
manufacturers for input and output capacitor selection.
Table 3. Capacitor Vendor Information
Supplier Phone Website
AVX (803) 448 - 9411 www.avxcorp.com
Sanyo (619) 661 - 6322 www.sanyovideo.com
TDK (847) 803 - 6100 www.component.tdk.com
Murata USA: [phone]
[phone] www.murata.com
Taiyo Yuden (408) 573 - 4150 www.t-yuden.com
Operating Frequency Selection
There are several considerations in selecting the operating
frequency of the converter. The first is staying clear of
sensitive frequency bands, which cannot tolerate any
spectral noise. For example in products incorporating RF
communications the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz and in
that case a 1.5MHz switching converter frequency may be
employed. The second consideration is the physical size of
the converter. As the operating frequency goes up, the
inductor and filter capacitors go down in value and size.
The trade off is in efficiency, since the switching losses due
to gate charge increase proportional with frequency.
Thermal Considerations
For the LTC3458L to deliver its full output power, it is
imperative that a good thermal path be provided to dissi-
pate the heat generated within the package. This can be
accomplished by taking advantage of the large thermal
pad on the underside of the IC. It is recommended that
multiple vias in the printed circuit board be used to
conduct heat away from the IC and into a copper plane with
as much area as possible. If the junction temperature rises
above ~150°C, the part will go into thermal shutdown, and
all switching will stop until the temperature drops.
Compensating the Feedback Loop
The LTC3458L uses current mode control, with internal
adaptive slope compensation. Current mode control elimi-
nates the 2nd order filter due to the inductor and output
capacitor exhibited in voltage mode controllers, and sim-
plifies the power loop to a single pole filter response. The
product of the modulator control to output DC gain, and
the error amp open-loop gain gives the DC gain of the
system:
GG G
V
V
G
G
V
I
GG
R
DC CONTROL EA
REF
OUT
CURRENT SENSE
CONTROL
IN
OUT
EA CURRENT SENSE
DS ON
=
=
≈ =
•• •
•
,
,
_
_
()
2
1000
1
The output filter pole is given by:
f
I
VC
FILTER POLE
OUT
OUT OUT
_
••
,=
π
where C
OUT
is the output filter capacitor.
The output filter zero is given by:
f
RC
FILTER ZERO
ESR OUT
_
••
,=
1
2π
where R
ESR
is the output capacitor equivalent series
resistance.
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP), and is given by:
f
V
IVL
RHPZ
IN
OUT OUT
=
2
2π •• •
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
rolled off before the RHP zero frequency.
APPLICATIO S I FOR ATIO
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