LT3505EMS8E#PBF Linear Technology, LT3505EMS8E#PBF Datasheet - Page 11

• 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
Finite transistor bandwidth limits the speed at which the
power switch can be turned on and off, effectively setting
the minimum on-time of the LT3505. For a given output volt-
age, the minimum on-time determines the maximum input
voltage to remain in continuous mode operation, V
See the “Input Voltage Range” section of the datasheet for
more information on determining V
frequencies below 750kHz, operation above V
to 40V) is safe provided that the system will tolerate the
pulse-skipping behavior outlined in the “Minimum On
Time” section of the datasheet. At switching frequencies
exceeding 750kHz, edge loss limits operation to input
voltages below V
Finite transition time results in a small amount of power
dissipation each time the power switch turns on and off
(edge loss). Edge loss increases with frequency, switch
current, and input voltage.
Input Voltage Frequency Foldback
In constant frequency operation (below V
loss only reduces the application effi ciency. However, at
high switching frequencies exceeding 750kHz and input
voltages exceeding V
ping mode and the switch current can increase above the
current limit of the part, 1.75A. This further increases the
power dissipated during switch transitions and increases
die temperature. To remedy the situation a single resis-
tor (R5) and a zener diode (D3) can be added to a typical
LT3505 circuit as shown in Figure 4.
6.7V TO 36V
BZT52C16T
ON OFF
V
D3
IN
1µF
806k
16V
R5
C2
IN(PS)
V
SHDN
R
IN(PS)
IN
T
R4
20.0k
.
Figure 4. 2.2MHz, 5V Application with Input Voltage Frequency Foldback Circuit
LT3505
, the part operates in pulse-skip-
GND
BOOST
SW
V
FB
C
R3
100k
IN(PS)
C4
22pF
. For switching
C3
0.1µF
D1
MBRM140
IN(PS)
6.8µH
IN(PS)
L1
1N4148
) edge
IN(PS)
D2
(up
R1
61.9k
R2
11.5k
.
When the input voltage is below 16V, the zener diode
path conducts no current and the current fl owing out
of the R
25µA, which programs a 2.2MHz switching frequency.
As the input voltage is increased above 16V, the zener
diode begins to conduct and gradually reduces the cur-
rent fl owing out of the R
the switching frequency as the input voltage is increased
above 16V (up to 36V) to ensure that the part constantly
operates in continuous mode without skipping pulses,
thereby preventing the excessive die temperature rise
encountered in pulse-skipping mode.
Although the circuit can be operated indefi nitely above
V
protect circuits during temporary periods of high input
voltage. For example, in many automotive systems, the
normal operating input range might be 9V to 16V, and
the LT3505 can be programmed to operate above the
AM band (>1.8MHz). At the same time, the circuit must
be able to withstand higher input voltages due to load
dump or double-battery jump starts. During these brief
periods, it is usually acceptable to switch at a frequency
within the AM band.
If the output is shorted while the input voltage is greater than
V
and the part will not be able to recover from the short until
the input voltage is reduced below V
discussion).
C5
22pF
ZENER
ZENER
3505 F04
, this frequency foldback method is intended to
, the switching frequency will be reduced to 30kHz
C1
10µF
V
5V
T
OUT
pin (and through R4) is nominally 0.5V/20k =
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
T
0
pin. This mechanism reduces
5
10
ZENER
Switching
Frequency
Maximum
Load Current
Input Voltage [V]
15
(see the following
20
LT3505
25
30
11
LTC3505 • F04b
35
3505fc
40