LT3505EMS8E#PBF Linear Technology, LT3505EMS8E#PBF Datasheet - Page 13

• 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
Inductor Selection and Maximum Output Current
A good fi rst choice for the inductor value is:
where V
L is in µH and f
be no subharmonic oscillation for applications with 50%
or greater duty cycle. The inductor’s RMS current rating
must be greater than your maximum load current and
its saturation current should be about 30% higher. For
robust operation in fault conditions, the saturation current
should be above 2.2A. To keep effi ciency high, the series
resistance (DCR) should be less than 0.1 . Table 2 lists
several vendors and types that are suitable.
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A
larger value provides a higher maximum load current and
reduces output voltage ripple at the expense of slower
transient response. If your load is lower than 1.2A, then
you can decrease the value of the inductor and operate
with higher ripple current. This allows you to use a physi-
cally smaller inductor, or one with a lower DCR resulting
in higher effi ciency. There are several graphs in the Typical
Performance Characteristics section of this data sheet that
show the maximum load current as a function of input
voltage and inductor value for several popular output volt-
ages. Low inductance may result in discontinuous mode
operation, which is okay, but further reduces maximum
load current. For details on maximum output current and
discontinuous mode operation, see Linear Technology
Application Note 44.
Table 2. Inductor Vendors
VENDOR
Sumida
Toko
Würth Elektronik
L = 1.2 (V
D
is the voltage drop of the catch diode (~0.4V),
OUT
SW
+ V
is in MHz. With this value there will
D
)/f
SW
URL
www.sumida.com
www.toko.com
www.we-online.com
PART SERIES
CDRH4D28
CDRH5D28
CDRH5D28
A916CY
D585LC
WE-TPC(M)
WE-PD2(M)
WE-PD(S)
Catch Diode
Depending on load current, a 1A to 2A Schottky diode is
recommended for the catch diode, D1. The diode must
have a reverse voltage rating equal to or greater than the
maximum input voltage. The ON Semiconductor MBRM140
is a good choice; it is rated for 1A continuous forward
current and a maximum reverse voltage of 40V.
Input Capacitor
The input of the LT3505 circuit must be bypassed with a
X7R or X5R type ceramic capacitor. Y5V types have poor
performance over temperature and applied voltage and
should not be used. For switching frequencies higher than
750kHz, bypass the input with a 1µF or higher value ceramic
capacitor. For switching frequencies below 750kHz, bypass
the input with a 2.2µF or higher value ceramic capacitor.
If the input power source has high impedance, or there is
signifi cant inductance due to long wires or cables, additional
bulk capacitance may be necessary. This can be provided
with a low performance electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3505 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
To accomplish this task, the input bypass capacitor must
be placed close to the LT3505 and the catch diode; see
the PCB Layout section. A second precaution regarding
the ceramic input capacitor concerns the maximum input
voltage rating of the LT3505. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (underdamped) tank circuit. If the LT3505 circuit
is plugged into a live supply, the input voltage can ring to
INDUCTANCE RANGE (µH)
1.2 to 4.7
2.5 to 10
2.5 to 33
1.1 to 39
2.2 to 22
2 to 12
1 to 10
1 to 27
Size (mm)
LT3505
4.5 × 4.5
5.5 × 5.5
8.3 × 8.3
6.3 × 6.2
8.1 × 8.0
4.8 × 4.8
5.2 × 5.8
7.3 × 7.3
13
3505fc