LTC3458_15 Datasheet, PDF (12/16 Pages) Linear Technology – 1.4A, 1.5MHz Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3458
APPLICATIO S I FOR ATIO
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.
Compensating the Feedback Loop
The LTC3458 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:
G_DC
=
G_CONTROL
• G _EA
•
V_REF
V_OUT
• G_{CURRENT _SENSE}
G_CONTROL
=
2 • V_IN
I_OUT
,
G_EA ≈ 1,000
G_{CURRENT _SENSE}
=
1
R_DS(ON)
The output filter pole is given by:
f_{FILTER _ POLE}
=
π
I_OUT
• V_OUT • C_OUT
,
where C_OUT is the output filter capacitor.
The output filter zero is given by:
f_{FILTER _ ZERO}
=
2π
1
• R_ESR
• C_OUT
,
where R_ESR is the output capacitor equivalent series
resistance.
Thermal Considerations
For the LTC3458 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.
A troublesome feature of the boost regulator topology is
the right half plane zero (RHP), and is given by:
2
f_RHPZ
=
2π
V_IN
•I_OUT •
V_OUT
•L
At heavy loads this gain increase with phase lag can occur
at a relatively low frequency. The loop gain is typically
3458fa
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