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LTC3407EDD-2#PBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16
LTC3407-2
7
34072fc
OPERATION
Low Current Operation
Two modes are available to control the operation of the
LTC3407-2 at low currents. Both modes automatically
switch from continuous operation to the selected mode
when the load current is low.
To optimize effi ciency, the Burst Mode operation can be
selected. When the load is relatively light, the LTC3407-2
automatically switches into Burst Mode operation, in which
the PMOS switch operates intermittently based on load
demand with a fi xed peak inductor current. By running
cycles periodically, the switching losses which are domi-
nated by the gate charge losses of the power MOSFETs
are minimized. The main control loop is interrupted when
the output voltage reaches the desired regulated value.
A hysteretic voltage comparator trips when I
TH
is below
0.35V, shutting off the switch and reducing the power. The
output capacitor and the inductor supply the power to the
load until I
TH
exceeds 0.65V, turning on the switch and the
main control loop which starts another cycle.
For lower ripple noise at low currents, the pulse-skipping
mode can be used. In this mode, the LTC3407-2 continues
to switch at a constant frequency down to very low cur-
rents, where it will begin skipping pulses. The effi ciency in
pulse-skipping mode can be improved slightly by connect-
ing the SW node to the MODE/SYNC input which reduces
the clock frequency by approximately 30%.
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases to 100% which
is the dropout condition. In dropout, the PMOS switch is
turned on continuously with the output voltage being equal
to the input voltage minus the voltage drops across the
internal P-channel MOSFET and the inductor.
An important design consideration is that the R
DS(ON)
of the P-channel switch increases with decreasing input
supply voltage (see Typical Performance Characteristics).
Therefore, the user should calculate the power dissipation
when the LTC3407-2 is used at 100% duty cycle with low
input voltage (see Thermal Considerations in the Applica-
tions Information section).
Low Supply Operation
To prevent unstable operation, the LTC3407-2 incorporates
an undervoltage lockout circuit which shuts down the part
when the input voltage drops below about 1.65V.
APPLICATIONS INFORMATION
A general LTC3407-2 application circuit is shown in
Figure 2. External component selection is driven by the
load requirement, and begins with the selection of the
inductor L. Once the inductor is chosen, C
IN
and C
OUT
can be selected.
Inductor Selection
Although the inductor does not infl uence the operating
frequency, the inductor value has a direct effect on ripple
current. The inductor ripple current ΔI
L
decreases with
higher inductance and increases with higher V
IN
or V
OUT
:
I
L
=
V
OUT
f
O
•L
•1
V
OUT
V
IN
Accepting larger values of ΔI
L
allows the use of low
inductances, but results in higher output voltage ripple,
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is
ΔI
L
= 0.4 • I
OUT(MAX)
, where I
OUT(MAX)
is 800mA. The
largest ripple current ΔI
L
occurs at the maximum input
voltage. To guarantee that the ripple current stays below a
specifi ed maximum, the inductor value should be chosen
according to the following equation:
L
V
OUT
f
O
• I
L
•1–
V
OUT
V
IN(MAX)
The inductor value will also have an effect on Burst Mode
operation. The transition from low current operation
begins when the peak inductor current falls below a level
set by the burst clamp. Lower inductor values result in
higher ripple current which causes this to occur at lower
load currents. This causes a dip in effi ciency in the upper
LTC3407-2
8
34072fc
APPLICATIONS INFORMATION
range of low current operation. In Burst Mode operation,
lower inductance values will cause the burst frequency
to increase.
Inductor Core Selection
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. Toroid
or shielded pot cores in ferrite or permalloy materials are
small and do not radiate much energy, but generally cost
more than powdered iron core inductors with similar elec-
trical characteristics. The choice of which style inductor
to use often depends more on the price vs size require-
ments and any radiated fi eld/EMI requirements than on
what the LTC3407-2 requires to operate. Table 1 shows
some typical surface mount inductors that work well in
LTC3407-2 applications.
Input Capacitor (C
IN
) Selection
In continuous mode, the input current of the converter is a
square wave with a duty cycle of approximately V
OUT
/V
IN
.
To prevent large voltage transients, a low equivalent series
resistance (ESR) input capacitor sized for the maximum
RMS current must be used. The maximum RMS capacitor
current is given by:
I
RMS
≈I
MAX
V
OUT
(V
IN
–V
OUT
)
V
IN
where the maximum average output current I
MAX
equals
the peak current minus half the peak-to-peak ripple cur-
rent, I
MAX
= I
LIM
– ΔI
L
/2.
This formula has a maximum at V
IN
= 2V
OUT
, where I
RMS
= I
OUT
/2. This simple worst-case is commonly used to
design because even signifi cant deviations do not offer
much relief. Note that capacitor manufacturer’s ripple cur-
rent ratings are often based on only 2000 hours lifetime.
This makes it advisable to further derate the capacitor,
or choose a capacitor rated at a higher temperature than
required. Several capacitors may also be paralleled to meet
the size or height requirements of the design. An additional
0.1μF to 1μF ceramic capacitor is also recommended on
V
IN
for high frequency decoupling, when not using an all
ceramic capacitor solution.
Table 1. Representative Surface Mount Inductors
PART
NUMBER
VALUE
(μH)
DCR
(Ω MAX)
MAX DC
CURRENT (A)
SIZE
W × L × H (mm
3
)
Sumida
CDRH3D16
2.2
3.3
4.7
0.075
0.110
0.162
1.20
1.10
0.90
3.8 × 3.8 × 1.8
Sumida
CDRH2D11
1.5
2.2
0.068
0.170
0.900
0.780
3.2 × 3.2 × 1.2
Sumida
CMD4D11
2.2
3.3
0.116
0.174
0.950
0.770
4.4 × 5.8 × 1.2
Murata
LQH32CN
1.0
2.2
0.060
0.097
1.00
0.79
2.5 × 3.2 × 2.0
Toko
D312F
2.2
3.3
0.060
0.260
1.08
0.92
2.5 × 3.2 × 2.0
Panasonic
ELT5KT
3.3
4.7
0.17
0.20
1.00
0.95
4.5 × 5.4 × 1.2
Output Capacitor (C
OUT
) Selection
The selection of C
OUT
is driven by the required ESR to
minimize voltage ripple and load step transients. Typically,
once the ESR requirement is satisfi ed, the capacitance
is adequate for fi ltering. The output ripple (ΔV
OUT
) is
determined by:
V
OUT
I
L
ESR+
1
8f
O
C
OUT
where f = operating frequency, C
OUT
= output capacitance
and ΔI
L
= ripple current in the inductor. The output ripple
is highest at maximum input voltage since ΔI
L
increases
with input voltage. With ΔI
L
= 0.4 • I
OUT(MAX)
, the output
ripple will be less than 100mV at maximum V
IN
and f
O
=
2.25MHz with:
ESR
COUT
< 150mΩ
Once the ESR requirements for C
OUT
have been met, the
RMS current rating generally far exceeds the I
RIPPLE(P-P)
requirement, except for an all ceramic solution.
In surface mount applications, multiple capacitors may
have to be paralleled to meet the capacitance, ESR or
RMS current handling requirement of the application.
Aluminum electrolytic, special polymer, ceramic and dry
tantalum capacitors are all available in surface mount
packages. The OS-CON semiconductor dielectric capacitor
available from Sanyo has the lowest ESR (size) product
of any aluminum electrolytic at a somewhat higher price.
Special polymer capacitors, such as Sanyo POSCAP,
Panasonic Special Polymer (SP), and Kemet A700, of-
LTC3407-2
9
34072fc
APPLICATIONS INFORMATION
Figure 2. LTC3407-2 General Schematic
fer very low ESR, but have a lower capacitance density
than other types. Tantalum capacitors have the highest
capacitance density, but they have a larger ESR and it
is critical that the capacitors are surge tested for use in
switching power supplies. An excellent choice is the AVX
TPS series of surface mount tantalums, available in case
heights ranging from 2mm to 4mm. Aluminum electrolytic
capacitors have a signifi cantly larger ESR, and are often
used in extremely cost-sensitive applications provided that
consideration is given to ripple current ratings and long
term reliability. Ceramic capacitors have the lowest ESR
and cost, but also have the lowest capacitance density,
a high voltage and temperature coeffi cient, and exhibit
audible piezoelectric effects. In addition, the high Q of
ceramic capacitors along with trace inductance can lead
to signifi cant ringing.
In most cases, 0.1μF to 1μF of ceramic capacitors should
also be placed close to the LTC3407-2 in parallel with the
main capacitors for high frequency decoupling.
Ceramic Input and Output Capacitors
Higher value, lower cost ceramic capacitors are now be-
coming available in smaller case sizes. These are tempting
for switching regulator use because of their very low ESR.
Unfortunately, the ESR is so low that it can cause loop
stability problems. Solid tantalum capacitor ESR generates
a loop “zero” at 5kHz to 50kHz that is instrumental in giving
acceptable loop phase margin. Ceramic capacitors remain
capacitive to beyond 300kHz and usually resonate with their
ESL before ESR becomes effective. Also, ceramic caps are
prone to temperature effects which requires the designer
to check loop stability over the operating temperature
RUN2 V
IN
V
IN
= 2.5V
TO 5.5V
V
OUT2
V
OUT1
RUN1
POR
SW1
V
FB1
GND
V
FB2
SW2
MODE/SYNC
LTC3407-2
C
IN
R5
POWER-ON
RESET
C4C5
L1
L2
R4 R2
R1
R3
C
OUT2
C
OUT1
3407 F02
PS*
BM*
*MODE/SYNC = 0V: PULSE-SKIPPING
MODE/SYNC = V
IN
: Burst Mode OPERATION
range. To minimize their large temperature and voltage
coeffi cients, only X5R or X7R ceramic capacitors should
be used. A good selection of ceramic capacitors is available
from Taiyo Yuden, AVX, Kemet, TDK and Murata.
Great care must be taken when using only ceramic input
and output capacitors. When a ceramic capacitor is used
at the input and the power is being supplied through long
wires, such as from a wall adapter, a load step at the output
can induce ringing at the V
IN
pin. At best, this ringing can
couple to the output and be mistaken as loop instability.
At worst, the ringing at the input can be large enough to
damage the part.
Since the ESR of a ceramic capacitor is so low, the input
and output capacitor must instead fulfi ll a charge storage
requirement. During a load step, the output capacitor must
instantaneously supply the current to support the load
until the feedback loop raises the switch current enough
to support the load. The time required for the feedback
loop to respond is dependent on the compensation and
the output capacitor size. Typically, three to four cycles are
required to respond to a load step, but only in the fi rst cycle
does the output drop linearly. The output droop, V
DROOP
,
is usually about two to three times the linear drop of the
fi rst cycle. Thus, a good place to start is with the output
capacitor size of approximately:
C
OUT
≈ 2.5
ΔI
OUT
f
O
•V
DROOP
More capacitance may be required depending on the duty
cycle and load step requirements.
In most applications, the input capacitor is merely required
to supply high frequency bypassing, since the impedance
to the supply is very low. A 10μF ceramic capacitor is
usually enough for these conditions.
Setting the Output Voltage
The LTC3407-2 develops a 0.6V reference voltage between
the feedback pin, V
FB
, and the ground as shown in Figure 2.
The output voltage is set by a resistive divider according
to the following formula:
V
OUT
= 0.6V 1+
R2
R
1
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P16

LTC3407EDD-2#PBF

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Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators Dual, Sync. 800mA 2.25MHz Step-dwn Convrtr in DFN
Lifecycle:
New from this manufacturer.
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