LT3505EMS8E#PBF Linear Technology, LT3505EMS8E#PBF Datasheet - Page 19

• Manufacturer Part Number: LT3505EMS8E#PBF
• Description: IC REG SW STP DWN 1.2A 8-MSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3505EMS8E#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 0.78 ~ 33.8 V
• Current - Output: 1.2A
• Frequency - Switching: 200kHz ~ 3MHz
• Voltage - Input: 3.6 ~ 36 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-MSOP Exposed Pad, 8-HMSOP, 8-eMSOP
• Dc To Dc Converter Type: Step Down
• Pin Count: 8
• Input Voltage: 36V
• Output Voltage: 0.78 to 33.84V
• Switching Freq: 200 TO 3000KHz
• Output Current: 1.2A
• Package Type: MSOP EP
• Output Type: Adjustable
• Switching Regulator: Yes
• Line Regulation: 0.007%/V(Typ)
• Mounting: Surface Mount
• Input Voltage (min): 3.6V
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -
• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
capacitor (C
used. In addition, a lower value fi lter capacitor (C
added in parallel. The fi lter capacitor is not a part of the loop
compensation but is used to fi lter 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 amplifi er with
fi nite output impedance. The power section, consisting of
the modulator, power switch and inductor, is modeled as
a transconductance amplifi er generating an output cur-
rent proportional to the voltage at the V
the output capacitor integrates this current and that the
capacitor on the V
fi er output current, resulting in two poles in the loop. R
provides a zero. With the recommended output capacitor,
the loop crossover occurs above the R
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
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.
C
) and a resistor (R
C
node (C
PL
) across the feedback divider may
C
) integrates the error ampli-
C
) in series to ground are
C
C
C
C
zero. This simple
node. Note that
F
) may be
C
Figure 12. A Good PCB Layout Ensures Proper, Low EMI Operation
C
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
fl ow in the LT3505’s V
and the input capacitor (C2). The loop formed by these
components should be as small as possible and tied to
F
LT3505
GROUND
SYSTEM
V
R
C
0.8V
C
C
D1
C
GROUND
POWER
Figure 11. Model for Loop Response
C1
V
2M
AMPLIFIER
OUT
–
+
ERROR
1.1A/V
SW
g
m
200µA/V
=
CURRENT MODE
C2
POWER STAGE
g
m
IN
=
–
+
and SW pins, the catch diode (D1)
V
: VIAS TO LOCAL GROUND PLANE
: OUTLINE OF LOCAL GROUND PLANE
IN
BOOST
SHUTDOWN
780mV
SW
1
2
3
4
FB
V
OUT
GROUND
SIGNAL
R1
R2
8
7
6
5
V
C
C
ESR
C1
LT3505
R
PL
T
+
FB
3505 F12
3505 F11
OUT
19
C1
3505fc