LT3505EMS8E#PBF Linear Technology, LT3505EMS8E#PBF Datasheet - Page 16

• 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

LT3505
APPLICATIONS INFORMATION
capacitor and a small Schottky diode (such as the BAT-54).
For lower output voltages tie a Schottky diode to the input
(Figure 6b). The circuit in Figure 6a is more effi cient because
the BOOST pin current comes from a lower voltage source.
You must also be sure that the maximum voltage rating
of the BOOST pin is not exceeded.
The minimum operating voltage of an LT3505 applica-
tion is limited by the undervoltage lockout (3.6V) and by
the maximum duty cycle as outlined above. For proper
start-up, the minimum input voltage is also limited by
the boost circuit. If the input voltage is ramped slowly,
or the LT3505 is turned on with its SHDN pin when the
output is already in regulation, then the boost capacitor
may not be fully charged. Because the boost capacitor is
charged with the energy stored in the inductor, the circuit
will rely on some minimum load current to get the boost
circuit running properly. This minimum load will depend
on the input and output voltages and on the arrangement
of the boost circuit. The minimum load generally goes to
zero once the circuit has started. Figure 7 shows a plot of
minimum load to start and to run as a function of input
voltage. In many cases the discharged output capacitor
will present a load to the switcher which will allow it to
start. The plots show the worst-case situation where V
is ramping verly slowly. For lower start-up voltage, the
boost diode can be tied to V
input range to one-half of the absolute maximum rating
16
(7a) Typical Minimum Input Voltage, V
7.2
7.0
6.8
6.6
6.4
6.2
6.0
5.8
5.6
5.4
5.2
1
TO START
TO RUN
LOAD CURRENT (mA)
10
IN
; however this restricts the
100
OUT
T
= 5V, f
A
= 25°C
3505 G15
1000
SW
= 750kHz
IN
Figure 7
of the BOOST pin.
At light loads, the inductor current becomes discontinu-
ous and the effective duty cycle can be very high. This
reduces the minimum input voltage to approximately
400mV above V
current is continuous and the duty cycle is limited by the
maximum duty cycle of the LT3505, requiring a higher
input voltage to maintain regulation.
Soft-Start
The SHDN pin can be used to soft-start the LT3505, reducing
the maximum input current during start-up. The SHDN pin
is driven through an external RC fi lter to create a voltage
ramp at this pin. Figure 8 shows the start-up waveforms
with and without the soft-start circuit. By choosing a large
RC time constant, the peak start up current can be reduced
to the current that is required to regulate the output, with
no overshoot. Choose the value of the resistor so that it
can supply 20µA when the SHDN pin reaches 2.3V.
Shorted and Reversed Input Protection
If the inductor is chosen so that it won’t saturate exces-
sively, an LT3505 buck regulator will tolerate a shorted
output. There is another situation to consider in systems
where the output will be held high when the input to the
LT3505 is absent. This may occur in battery charging ap-
(7b) Typical Minimum Input Voltage, V
3.9
3.7
3.5
5.5
5.3
5.1
4.9
4.7
4.5
4.3
4.1
1
OUT
T
TO START
A
TO RUN
= 25°C
. At higher load currents, the inductor
LOAD CURRENT (mA)
10
100
OUT
= 3.3V, f
1000
SW
= 750kHz
3505fc