lt3740 Linear Technology Corporation, lt3740 Datasheet - Page 13

• Manufacturer Part Number: lt3740
• Description: Wide Operating Range, Valley Mode, No Rsense Synchronous Step-down Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
Other losses, including C
conduction loss during dead time and inductor core loss
generally account for less than 2% additional loss.
When making adjustments to improve efficiency, the input
current is the best indicator of changes in efficiency. If a
change is made and the input current decreases, then the
efficiency has increased. If there is no change in input
current, then there is no change in efficiency.
Checking Transient Response
The regulator loop response can be checked by looking at
the load transient response. Switching regulators take
several cycles to respond to a step in load current. When
a load step occurs, V
equal to ΔI
resistance of C
charge C
regulator to return V
During this recovery time, V
overshoot or ringing that would indicate a stability
problem. The V
Figure 2 will provide adequate compensation for most
applications. For a detailed explanation of switching
control loop theory see Application Note 76.
Step-Up Converter Inductor Selection
The step-up converter in the LT3740 provides a BIAS
voltage about 7V higher than the input voltage V
top MOSFET drive and most of the internal controller
circuitry. The step-up converter has a current limit of
400mA. An inductor ripple current from 100mA to 200mA
is a reasonable design for the converter. For this consider-
ation, a 22μH or 47μH inductor is recommended for most
of the LT3740 applications. Small size and high efficiency
are the major concerns. Inductors with low core losses
and small DCR at 1MHz are good choices. Some induct-
ors in this category with small size are listed in
Table 2.
OUT
LOAD
generating a feedback error signal used by the
OUT
*(ESR), where ESR is the effective series
C
. ΔI
U
pin external components shown in
OUT
LOAD
OUT
immediately shifts by an amount
OUT
U
also begins to charge or dis-
ESR loss, Schottky diode D1
to its steady-state value.
OUT
W
can be monitored for
U
IN
for the
Table 2. Recommended Inductors for Step-Up Converter
Part Number
LQH3C220
ELT5KT-220
CDRH3D16-220
CR32-470
The step-up converter inductor current is the greatest
when the input voltage is the lowest. Larger Cgs of the
MOSFETs results in large inductor current. Connecting
the BGDP pin to the BIAS pin also greatly increases the
inductor current. The saturation current of the inductor
needs to cover the maximum input current.
The step-up inductor current decreases as the input volt-
age increases. For high input voltages, the step-up con-
verter will begin skipping pulses. Although this will result
in some low frequency ripple, the BIAS voltage remains
regulated on an average basis, and the step-down
controller operation is not affected.
For V
rent should be higher than 400mA. For V
a lower current rating inductor could be used. The inductor
RMS current should be higher than 250mA, and the
inductance should not be less than 10μH at 400mA.
Step-Up Converter Capacitor Selection
The small size of ceramic capacitors makes them ideal for
the output of the LT3740 step-up converter. X5R and X7R
types are recommended because they retain their capaci-
tance over wider voltage and temperature ranges than
other types such as Y5V or Z5U. A 1μF capacitor is
recommended for the output of the LT3740 step-up
converter.
Table 3. Recommended Ceramic Capacitor Manufacturers
Manufacturer
Taiyo Yuden
Murata
Kemet
IN
higher than 10V step-up inductor saturation cur-
DCR (Ω)
0.71
0.9
0.43
0.97
Phone
408-573-4150
814-237-1431
408-986-0424
Current
Rating (mA)
250
420
400
330
URL
www.t-yuden.com
www.murata.com
www.kemet.com
IN
Manufacturer
Murata
814-237-1431
www.murata.com
Panasonic
714-373-7334
www.panasonic.com
Sumida
847-956-0666
www.Sumida.com
lower than 10V,
LT3740
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3740fa