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LTC3458LEDE#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3458L
10
3458lfa
transition occurs using a single resistor. During Burst
Mode operation, the oscillator is shut down, since the on
time is determined by the time it takes the inductor current
to reach a fixed peak current, and the off time is deter-
mined by the time it takes for the inductor current to return
to zero.
In Burst Mode operation, the IC delivers energy to the
output until it is regulated and then goes into a sleep mode
where the outputs are off and the IC is consuming only
12μA of quiescent current. In this mode the output ripple
voltage has a variable frequency component with load
current and will be typically 2% peak-to-peak. This maxi-
mizes efficiency at very light loads by minimizing switch-
ing and quiescent losses. Burst Mode ripple can be re-
duced slightly by using more output capacitance (22μF or
greater). This capacitor does not need to be a low ESR type
if low ESR ceramics are also used. Another method of
reducing Burst Mode ripple is to place a small feed-
forward capacitor across the upper resistor in the V
OUT
feedback divider network.
During Burst Mode operation, COMP is disconnected
from the error amplifier in an effort to hold the voltage on
the external compensation network where it was before
entering Burst Mode operation. To minimize the effects of
leakage current and stray resistance, voltage clamps limit
the minimum and maximum voltage on COMP during
Burst Mode operation. This minimizes the transient expe-
rienced when a heavy load is suddenly applied to the
converter after being in Burst Mode operation for an
extended period of time.
For automatic operation, an RC network should be con-
nected from BURST to ground. The value of the resistor
will control the average load current (I
BURST
) at which
Burst Mode operation will be entered and exited (there is
hysteresis to prevent oscillation between modes). The
equation given for the capacitor on BURST is for the
minimum value, to prevent ripple on the BURST pin from
causing the part to oscillate in and out of Burst Mode
operation at the current where the mode transition occurs.
R
I
BURST
BURST
=
10
where R
BURST
is in kΩ and I
BURST
is in amps.
C
CV
BURST
OUT OUT
=
•
,10 000
where C
BURST(MIN)
and C
OUT
are in μF.
Note: the BURST pin only sources current based on
current delivered to V
OUT
through the P-channel MOSFET.
If current in the inductor is allowed to go negative (this can
occur at very light loads and high step-up ratios), the burst
threshold may become inaccurate, preventing the IC from
entering Burst Mode operation. For R
BURST
values greater
than 200k, a larger than recommended inductor value may
be needed to ensure positive inductor current and auto-
matic Burst Mode operation.
In the event that a sudden load transient causes the voltage
level on FB to drop by more than 4% from the regulation
value, an internal pull-up is applied to BURST, forcing the
part quickly out of Burst Mode operation. For optimum
transient response when going between Burst Mode op-
eration and PWM mode, Burst can be controlled manually
by the host. This way PWM mode can be commanded
before the load step occurs, minimizing output voltage
drop. Note that Burst Mode operation is inhibited during
start-up and soft-start.
Manual Control
For applications requiring fixed frequency operation at all
load currents, connect the BURST pin to V
OUT
through a
51k resistor. To force Burst Mode operation, ground the
BURST pin.
For applications where a large load step can be anticipated,
the circuit below can be used to reduce the voltage transient
on V
OUT
. Automatic operation is achieved when the exter-
nal P-channel MOSFET is off and fixed frequency operation
is commanded when the external P-channel MOSFET is on.
In shutdown, the P-channel MOSFET should be off.
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Figure 1
0.01μF
V
IN
BURST
133k
P-CHANNEL
MOSFET
HIGH: AUTO MODE
LOW: FIXED FREQUENCY
3458L FO1
LTC3458L
11
3458lfa
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3458L allows for
the use of small surface mount inductors. Since the
internal slope compensation circuit relies on the inductor’s
current slope and frequency, Table 1 should be used to
select an inductor value for a given frequency of operation
(± 25%). The recommended value will yield optimal tran-
sient performance while maintaining stable operation.
Inductor values larger than listed in Table 1 are permis-
sible to reduce the current ripple.
Table 1. Recommended Inductor Values
Frequency Inductor Value(μH)
1.5MHz 3.3 to 4.7
1.25MHz 4.7 to 6.8
1MHz 6.8 to 10
750kHz 10 to 15
500kHz 15 to 22
For high efficiency, choose an inductor with high fre-
quency core material, such as ferrite, to reduce core
losses. The inductor should have low ESR (equivalent
series resistance) to reduce the I
2
R losses, and must be
able to handle the peak inductor current without saturat-
ing. Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents in the
1A to 3A region. To minimize radiated noise, use a
toroidal or shielded inductor. (Note that the inductance of
shielded types will drop more as current increases, and
will saturate more easily). See Table 2 for a list of inductor
manufacturers.
Table 2. Inductor Vendor Information
Supplier Phone Website
Coilcraft (847) 639-6400 www.coilcraft.com
TDK (847) 803-6100 www.component.tdk.com
Murata USA: (814) 237-1431
(800) 831-9172 www.murata.com
Sumida USA: (847) 956-0666
Japan: 81-3-3607-5111 www.japanlink.com/sumida
COEV (800) 227-7040 www.coev.net
Toko (847) 297-0070 www.tokoam.com
Wurth (202) 785-8800 www.we-online.com
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Some example inductor part types are:
Coilcraft: MSS7341 and MSS5131 Series
TDK: RLF5018T and SLF7045 Series
Murata: LQH4C and LQN6C Series
Sumida: CDRH4D28 and CDRH6D28 Series
COEV: DQ7545 Series
TOKO: D62CB and D63LCB Series
WURTH: WE-PD2 Series
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The max ripple due
to charge is given by:
V
IV
CVf
RBULK
PIN
OUT OUT
=
•
••
where I
P
= peak inductor current and f = switching
frequency.
The ESR (equivalent series resistance) is usually the most
dominant factor for ripple in most power converters. The
ripple due to capacitor ESR is given by:
V
RCESR
= I
P
• C
ESR
where C
ESR
= Capacitor Series Resistance.
The ESL (equivalent series inductance) is also an impor-
tant factor for high frequency converters. Using small,
surface mount ceramic capacitors, placed as close as
possible to the V
OUT
pins, will minimize ESL.
Low ESR/ESL capacitors should be used to minimize
output voltage ripple. For surface mount applications, AVX
TPS Series tantalum capacitors, Sanyo POSCAP, or Taiyo
Yuden X5R type ceramic capacitors are recommended.
For through-hole applications, Sanyo OS-CON capacitors
offer low ESR in a small package size.
In all applications, a minimum of 4.7μF (generally 22μF is
recommended), low ESR ceramic capacitor should be
placed as close to the V
OUT
pin as possible, and grounded
to a local ground plane.
..
..
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: (814) 237-1431
(800) 831-9172 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.
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LTC3458LEDE#PBF

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Buy LTC3458LEDE#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 1.7A, 1.5MHz Synch Boost Converter in DFN
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