lt3742 Linear Technology Corporation, lt3742 Datasheet - Page 17

• Manufacturer Part Number: lt3742
• Description: Dual, 2-phase Step-down Switching Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT3742’s control loop. Output ripple can be estimated
with the following equation:
where ΔI
switching frequency (500kHz). The ESR is so low for ce-
ramic capacitors that it can be left out of the above calcula-
tion. The output voltage ripple will be highest at maximum
input voltage (ΔI
shows several low-ESR capacitor manufacturers.
Table 3. Low ESR Surface Mount Capacitors
VENDOR
Taiyo Yuden
www.t-yuden.com
Murata
www.murata.com
Kemet
www.kemet.com
Sanyo
www.sanyo.com
Panasonic
www.panasonic.com
TDK
www.tdk.com
Nippon Chemicon
www.chemi-con.co.jp
Setting Output Voltage
The output of a bipolar controller requires a minimum load
to prevent current sourced from the switch pin charging
the output capacitor above the desired output voltage.
This current, approximately 5mA, may be accounted for
in the feedback string or the user may choose to force a
minimum load in their application.
V
RIPPLE
L
is the inductor ripple current and f
= Δ
I
L
L
⎛
⎜
⎝
8 •
increases with input voltage). Table 3
TYPE
Ceramic X5R, X7R
Ceramic X5R, X7R
Tantalum
Ta Organic
Al Organic
Ta or Al Organic
Al Organic
Ceramic X5R, X7R
Ceramic X5R, X7R
f
SW
1
•
C
OUT
+
ESR
Figure 7. Setting Output Voltage with the FB Pin
SERIES
T491, T494, T495
T520
A700
POSCAP
SP CAP
⎞
⎟
⎠
SW
V
OUT1
R1B
R1A
is the
V
FB1
LT3742
V
3742 F07
The output voltage for each controller is programmed
with a resistor divider between the output and the FB pin.
Always use 1% resistors (or better) for the best output
voltage accuracy. The value of R
and the value of R1 should be chosen according to:
Output Short-Circuit Protection
Because the LT3742 constantly monitors the inductor
current, both devices inherently providing excellent output
short-circuit protection. The N-channel MOSFET is not al-
lowed to turn on unless the inductor current is below the
threshold of the current sense comparator. This guaran-
tees that the inductor current will not “run away” and the
controller will skip cycles until the inductor current has
dropped below the current sense threshold.
Loop Compensation
An external resistor and capacitor connected in series from
the V
controller. Sometimes a second, smaller valued capacitor
is placed in parallel to fi lter switching frequency noise from
the V
and transient performance of each controller.
A practical approach is to start with values of R
and C
optimize the performance. When adjusting these values,
change only one value at a time (R
the transient response is affected. The simplest way to
check loop stability is to apply a load current step while
observing the transient response at the output. Stability
should then be checked across all operating conditions,
including load current, input voltage, and temperature to
ensure a robust design.
FB2
R
B
V
C
C
OUT2
C
=
pin to ground provides loop compensation for each
pin. Loop compensation determines the stability
R2B
R2A
= 330pF, then tune the compensation network to
R
A
•
⎛
⎜
⎝
V
0 8
OUT
.
V
–
1
⎞
⎟
⎠
A
C
should be 8k or less,
or C
C
), then see how
LT3742
C
17
= 10k
3742f