LT3435EFE#PBF Linear Technology, LT3435EFE#PBF Datasheet - Page 21

• Manufacturer Part Number: LT3435EFE#PBF
• Description: IC REG SW HV 3A 500KHZ 16-TSSOP
• Manufacturer: Linear Technology
• Type: Step-Down (Buck)r

Specifications of LT3435EFE#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: 1.25 ~ 54 V
• Current - Output: 3A
• Frequency - Switching: 500kHz
• Voltage - Input: 3.3 ~ 60 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Primary Input Voltage: 60V
• No. Of Outputs: 1
• Output Voltage: 68V
• Output Current: 2.4A
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

APPLICATIO S I FOR ATIO
Example: with V
Total power dissipation is:
Thermal resistance for the LT3435 package is influenced
by the presence of internal or backside planes. With a full
plane under the FE16 package, thermal resistance will be
about 45°C/W. No plane will increase resistance to about
150°C/W. To calculate die temperature, use the proper
thermal resistance number for the desired package and
add in worst-case ambient temperature:
With the FE16 package (Q
temperature of 70°C:
Input Voltage vs Operating Frequency Considerations
The absolute maximum input supply voltage for the LT3435
is specified at 60V. This is based solely on internal semi-
conductor junction breakdown effects. Due to internal
power dissipation the actual maximum V
particular application may be less than this.
A detailed theoretical basis for estimating internal power
loss is given in the section Thermal Considerations. Note
that AC switching loss is proportional to both operating
frequency and output current. The majority of AC switch-
ing loss is also proportional to the square of input voltage.
For example, while the combination of V
5V at 2A and f
multaneously raising V
possible. Nevertheless, input voltage transients up to 60V
can usually be accommodated, assuming the resulting
P
T
T
P
P
P
0 12 0 962 1 08
SW
BOOST
Q
J
J
TOT
.
= T
= 70 + 45(1.22) = 125°C
=
=
40 0 0026
+
= 1.08 + 0.03+ 0.11 = 1.22W
A
(
(
+ Q
0 15 2
=
.
.
.
( ) (
5
JA
OSC
)( ) ( )
25
2
IN
=
(P
40
240
= 500kHz may be easily achievable, si-
= 25V, V
)
TOT
2
.
U
/
+
5
5 0 001 0 11
)
)
IN
(
=
+
to 60V and f
.
U
0 03
(
77
OUT
.
JA
e
)
= 45°C/W) at an ambient
W
=
–
= 5V and I
9
)
.
( ) ( )( )(
W
12 2 25 500 3
/
OSC
W
IN
IN
to 700kHz is not
OUT
achievable in a
= 40V, V
= 2A:
U
e
OUT
)
=
increase in internal dissipation is of insufficient time dura-
tion to raise die temperature significantly.
A second consideration is controllability. A potential limi-
tation occurs with a high step-down ratio of V
as this requires a correspondingly narrow minimum switch
on time. An approximate expression for this (assuming
continuous mode operation) is given as follows:
where:
A potential controllability problem arises if the LT3435 is
called upon to produce an on time shorter than it is able to
produce. Feedback loop action will lower then reduce the
V
skipping or Burst Mode behavior is exhibited.
In summary:
1. Be aware that the simultaneous requirements of high
2. The simultaneous requirements of high V
FREQUENCY COMPENSATION
Before starting on the theoretical analysis of frequency
response the following should be remembered—the worse
the board layout, the more difficult the circuit will be to
stabilize. This is true of almost all high frequency analog
circuits. Read the Layout Considerations section first.
Common layout errors that appear as stability problems
are distant placement of input decoupling capacitor and/or
C
t
V
V
V
f
V
practice due to internal dissipation. The Thermal Con-
siderations section offers a basis to estimate internal
power. In questionable cases a prototype supply should
be built and exercised to verify acceptable operation.
high f
switch on time. Cycle skipping and/or Burst Mode be-
havior will result causing an increase in output voltage
ripple while maintaining the correct output voltage.
control voltage to the point where some sort of cycle-
ON(MIN)
OSC
IN
OUT
F
IN
= Schottky diode forward drop
, high I
= input voltage
= switching frequency
OSC
= output voltage
= V
can result in an unacceptably short minimum
OUT
OUT
and high f
+ V
F
/V
IN
(f
OSC
OSC
may not be achievable in
)
IN
LT3435
, low V
IN
to V
21
OUT
OUT
3435fa
and
,