LT3503 Datasheet, PDF (19/24 Pages) Linear Technology – 1A, 2.2MHz Step-Down Switching Regulator in 2mm × 3mm DFN

LT3505
APPLICATIONS INFORMATION
capacitor (C_C) and a resistor (R_C) in series to ground are
used. In addition, a lower value filter capacitor (C_F) may be
added in parallel. The filter capacitor is not a part of the loop
compensation but is used to filter noise at the switching
frequency, and is required only if a phase-lead capacitor
is used or if the output capacitor has high ESR.
Loop compensation determines the stability and transient
performance. Designing the compensation network is a bit
complicated and the best values depend on the application
and in particular the type of output capacitor. A practical
approach is to start with one of the circuits in this data
sheet that is similar to your application and tune the com-
pensation network to optimize the performance. Stability
should then be checked across all operating conditions,
including load current, input voltage and temperature. The
LT1375 data sheet contains a more thorough discussion of
loop compensation and describes how to test the stability
using a transient load.
Figure 11 shows an equivalent circuit for the LT3505 control
loop. The error amp is a transconductance amplifier with
finite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplifier generating an output cur-
rent proportional to the voltage at the V_C node. Note that
the output capacitor integrates this current and that the
capacitor on the V_C node (C_C) integrates the error ampli-
fier output current, resulting in two poles in the loop. R_C
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R_CC_C zero. This simple
model works well as long as the value of the inductor is
not too high and the loop crossover frequency is much
lower than the switching frequency. With a larger ceramic
capacitor (very low ESR), crossover may be lower and a
phase lead capacitor (C_PL) across the feedback divider may
improve the phase margin and transient response. Large
electrolytic capacitors may have an ESR large enough to
create an additional zero and the phase lead may not be
necessary.
If the output capacitor is different than the recommended
capacitor, stability should be checked across all operat-
ing conditions, including load current, input voltage and
temperature.
LT3505
0.8V
–
CURRENT MODE
POWER STAGE
g_m =
SW
+^1.1A/V
R1
FB
V_C
g_m =
200µA/V
ERROR
AMPLIFIER
780mV
2M
R2
OUT
C_PL
ESR
+
C1
C1
R_C
C_F
C_C
3505 F11
Figure 11. Model for Loop Response
PCB Layout
For proper operation and minimum EMI, care must be taken
during printed circuit board layout. Figure 12 shows the
recommended component placement with trace, ground
plane and via locations. Note that large, switched currents
flow in the LT3505’s V_IN and SW pins, the catch diode (D1)
and the input capacitor (C2). The loop formed by these
components should be as small as possible and tied to
SYSTEM
GROUND ^VOUT
C1
: VIAS TO LOCAL GROUND PLANE
: OUTLINE OF LOCAL GROUND PLANE
V_OUT
SW
D1
C2
POWER
GROUND
BOOST
1
2
3
4
8 _{VC FB}
7
6 R_T
5
SIGNAL
GROUND
3505 F12
V_IN SHUTDOWN
Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation
3505fc
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