Datasheet

LT3042
15
3042fb
For more information www.linear.com/LT3042
APPLICATIONS INFORMATION
the SET pin creates a 0.1% error in the reference voltage.
Leakages of this magnitude, coupled with other sources
of leakage, can cause significant errors in the output volt
-
age, especially over wide operating temperature range.
Figure2illustrates a typical guard ring layout technique.
requirements. While the LT3042 is robust enough not
to oscillate if the recommended layout is not followed,
depending on the actual layout, phase/gain margin, noise
and PSRR performance may degrade.
Stability and Output Capacitance
The LT3042 requires an output capacitor for stability.
Given its high bandwidth, LTC recommends low ESR and
ESL ceramic capacitors. A minimum 4.7µF output capaci
-
tor with an ESR below 50mΩ and an ESL below 2nH is
required for stability.
Given the high PSRR and low noise performance attained
using a single 4.7µF ceramic output capacitor, larger values
of output capacitor only marginally improves the perfor
-
mance because the regulator bandwidth decreases with
increasing output capacitance
—
hence, there is little to
be gained by using larger than the minimum 4.7µF output
capacitor. Nonetheless, larger values of output capacitance
do decrease peak output deviations during a load transient.
Note that bypass capacitors used to decouple individual
components powered by the LT3042 increase the effective
output capacitance.
Give extra consideration to the type of ceramic capacitors
used. They are manufactured with a variety of dielectrics,
each with different behavior across temperature and applied
voltage. The most common dielectrics used are specified
with EIA temperature characteristic codes of Z5U, Y5V,
3042 F02
11
OUT
SET
10
9
6
7
8
4
5
3
2
1
Figure2. Guard Ring Layout
Figure3. C
OUT
and C
SET
Connections for Stability
Since the SET pin is a high impedance node, unwanted
signals may couple into the SET pin and cause erratic
behavior. This is most noticeable when operating with a
minimum output capacitor at heavy load currents. By
-
passing the SET pin with a small capacitance to ground
resolves this issue — 10nF is sufficient.
For applications requiring higher accuracy or an adjust
-
able output voltage, the SET pin may be actively driven
by an external voltage source capable of sinking
100µA.
Connecting a precision voltage reference to the SET pin
eliminates any errors present in the output voltage due
to the reference current and SET pin resistor tolerances.
Output Sensing and Stability
The LT3042’s OUTS pin provides a Kelvin sense connection
to the output. The SET pin resistor’s GND side provides a
Kelvin sense connection to the load’s GND side.
Additionally, for ultrahigh PSRR, the LT3042 bandwidth
is made quite high (~1MHz), making it close to a typical
4.7µF (1206 case size) ceramic output capacitor’s self-
resonance frequency (~2.3MHz). Therefore, it is very
important to avoid adding extra impedance (ESR and
ESL) outside the feedback loop. To that end, as shown
in Figure3, minimize the effects of PCB trace and solder
inductance by tying the OUTS pin directly to C
OUT
and the
GND side of CSET directly to the GND side of C
OUT
, as
well as keep the GND sides of C
IN
and C
OUT
reasonably
close. Refer to the LT3042 demo board manual for more
information on the recommended layout that meets these
C
OUT
R
SET
C
SET
C
IN
OUT
IN
SET
LT3042
DEMO BOARD
PCB LAYOUT
ILLUSTRATES
4-TERMINAL
CONNECTION
TO C
OUT
100µA
OUTS
PG
ILIM
3042 F03
GND
PGFB
EN/UV
V
OUT
I
OUT(MAX)
: 200mA
V
IN
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