Datasheet

LT3742
17
3742fa
LT3742’s control loop. Output ripple can be estimated
with the following equation:
V
RIPPLE
= ΔI
L
1
8 • f
SW
• C
OUT
+ESR
⎛
⎝
⎜
⎞
⎠
⎟
where ΔI
L
is the inductor ripple current and f
SW
is the
switching frequency (500kHz). The ESR is so low for
ceramic capacitors that it can be left out of the above
calculation. The output voltage ripple will be highest at
maximum input voltage (ΔI
L
increases with input voltage).
Table 3 shows several low-ESR capacitor manufacturers.
Table 3. Low ESR Surface Mount Capacitors
VENDOR TYPE SERIES
Taiyo Yuden
www.t-yuden.com
Ceramic X5R, X7R
Murata
www.murata.com
Ceramic X5R, X7R
Kemet
www.kemet.com
Tantalum
Ta Organic
Al Organic
T491, T494, T495
T520
A700
Sanyo
www.sanyo.com
Ta or Al Organic POSCAP
Panasonic
www.panasonic.com
Al Organic SP CAP
TDK
www.tdk.com
Ceramic X5R, X7R
Nippon Chemicon
www.chemi-con.co.jp
Ceramic X5R, X7R
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.
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
A
should be 8k or less,
and the value of R1 should be chosen according to:
R
B
= R
A
•
V
OUT
0.8V
– 1
⎛
⎝
⎜
⎞
⎠
⎟
Output Short-Circuit Protection
Because the LT3742 constantly monitors the inductor cur-
rent, both devices inherently providing excellent output
short-circuit protection. The N-channel MOSFET is not
allowed 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
C
pin to ground provides loop compensation for each
controller. Sometimes a second, smaller valued capacitor
is placed in parallel to filter switching frequency noise from
the V
C
pin. Loop compensation determines the stability
and transient performance of each controller.
A practical approach is to start with values of R
C
= 10k
and C
C
= 330pF, then tune the compensation network to
optimize the performance. When adjusting these values,
change only one value at a time (R
C
or C
C
), then see how
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.
applicaTions inForMaTion
Figure 7. Setting Output Voltage with the FB Pin
LT3742
V
FB2
V
OUT2
V
OUT1
V
FB1
R2B
R2A
3742 F07
R1B
R1A