Datasheet

LT3740
13
3740fc
APPLICATIONS INFORMATION
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
OUT
immediately shifts by an amount
equal to ΔI
LOAD
*(ESR), where ESR is the effective series
resistance of C
OUT
. ΔI
LOAD
also begins to charge or
discharge C
OUT
generating a feedback error signal used
by the regulator to return V
OUT
to its steady-state value.
During this recovery time, V
OUT
can be monitored for
overshoot or ringing that would indicate a stability problem.
The V
C
pin external components shown in 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
IN
for the 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 consideration,
a 22µH or 47µH inductor is recommended for most of the
LT3740 applications. Small size and high effi ciency are the
major concerns. Inductors with low core losses and small
DCR at 1MHz are good choices. Some inductors in this
category with small size are listed in Table 2.
Table 2. Recommended Inductors for Step-Up Converter
PART NUMBER DCR (Ω)
CURRENT
RATING (MA) MANUFACTURER
LQH3C220 0.71 250 Murata
814-237-1431
www.murata.com
ELT5KT-220 0.9 420 Panasonic
714-373-7334
www.panasonic.com
CDRH3D16-220
CR32-470
0.43
0.97
400
330
Sumida
847-956-0666
www.Sumida.com
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 voltage
increases. For high input voltages, the step-up converter
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 opera-
tion is not affected.
For V
IN
higher than 10V step-up inductor saturation
current should be higher than 400mA. For V
IN
lower than
10V, 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 capacitance
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 PHONE URL
Taiyo Yuden 408-573-4150 www.t-yuden.com
Murata 814-237-1431 www.murata.com
Kemet 408-986-0424 www.kemet.com
Design Example
As a design example, take a supply with the following
specifi cations: V
IN
= 7V to 20V (15V nominal), V
OUT
= 2.5V
± 5%, I
OUT(MAX)
= 10A. First, choose the inductor for about
30% ripple current at nominal V
IN
:
L =
2.5V
(300kHz) • 0.3 • 10A
•1–
2.5V
15V
⎛
⎝
⎜
⎞
⎠
⎟
= 2.3μH