LT3575EFE#PBF Linear Technology, LT3575EFE#PBF Datasheet - Page 9

• Manufacturer Part Number: LT3575EFE#PBF
• Description: IC FLYBACK CONVERTER 16TSSOP
• Manufacturer: Linear Technology
• Type: Flyback, Isolatedr

Specifications of LT3575EFE#PBF

• Internal Switch(s): Yes
• Synchronous Rectifier: No
• Number Of Outputs: 1
• Voltage - Output: Adj to 60V
• Current - Output: 2.5A
• Frequency - Switching: 1MHz
• Voltage - Input: 3 ~ 40 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: *
• Package / Case: 16-TSSOP Exposed Pad, 16-eTSSOP, 16-HTSSOP
• Voltage - Supply: 3 V ~ 40 V
• Frequency-max: 1MHz
• Pwm Type: Regulator
• Buck: No
• Boost: No
• Flyback: Yes
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 3.5A
• No. Of Pins: 16
• Operating Temperature Range: -40°C To +125°C
• Msl: MSL 1 - Unlimited
• Peak Reflow Compatible (260 C): Yes
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Duty Cycle: -

APPLICATIONS INFORMATION
ERROR AMPLIFIER—DYNAMIC THEORY
Due to the sampling nature of the feedback loop, there
are several timing signals and other constraints that are
required for proper LT3575 operation.
Minimum Current Limit
The LT3575 obtains output voltage information from the
SW pin when the secondary winding conducts current.
The sampling circuitry needs a minimum amount of time
to sample the output voltage. To guarantee enough time,
a minimum inductance value must be maintained. The
primary side magnetizing inductance must be chosen
above the following value:
The minimum current limit is higher than that on the Elec-
trical Characteristics table due to the overshoot caused by
the comparator delay.
Leakage Inductance Blanking
When the output switch fi rst turns off, the fl yback pulse
appears. However, it takes a fi nite time until the transformer
primary side voltage waveform approximately represents
the output voltage. This is partly due to the rise time on
the SW node, but more importantly due to the trans-
former leakage inductance. The latter causes a very fast
voltage spike on the primary side of the transformer that
is not directly related to output voltage (some time is also
required for internal settling of the feedback amplifi er
circuitry). The leakage inductance spike is largest when
the power switch current is highest.
In order to maintain immunity to these phenomena, a fi xed
delay is introduced between the switch turn-off command
and the beginning of the sampling. The blanking is internally
set to 150ns. In certain cases, the leakage inductance may
not be settled by the end of the blanking period, but will
not signifi cantly affect output regulation.
t
I
L
MIN
MIN
PRI
= minimum off-time, 350ns
= minimum current limit, 400mA
V
OUT
•
t
I
MIN
MIN
•
N
PS
V
OUT
•
N
PS
•
. 0 88
V
µH
Selecting R
The expression for V
can be rearranged to yield the following expression for R
where,
The equation assumes the temperature coeffi cients of
the diode and V
approximation.
Strictly speaking, the above equation defi nes R
an absolute value, but as a ratio of R
question is, “What is the proper value for R
answer is that R
LT3575 is trimmed and specifi ed using this value of R
If the impedance of R
additional errors will result. However, a variation in R
several percent is acceptable. This yields a bit of freedom
in selecting standard 1% resistor values to yield nominal
R
should also be verifi ed experimentally, and adjusted if
necessary for best output accuracy.
Tables 1-4 are useful for selecting the resistor values for
R
R
common winding ratios.
Table 1. Common Resistor Values for 1:1 Transformers
FB
REF
REF
V
V
V
N
V
R
OUT
TC
/R
OUT
F
PS
3.3
FB
12
15
20
= Ratio of Q1, IC to IE, typically 0.986
5
and R
= Switching diode forward voltage
and R
REF
(V)
= 0.55V
= Effective primary-to-secondary turns ratio
= Output voltage
R
ratios. The R
FB
REF
TC
FB
with no equations. The tables provide R
and R
values for common output voltages and
1.00
1.00
1.00
1.00
1.00
N
•
TC
REF
PS
N
are equal, which is a good fi rst-order
PS
OUT,
should be approximately 6.04k. The
REF
REF
FB
developed in the Operation section,
V
Resistor Values
R
varies considerably from 6.04k,
OUT
V
FB
resistor given by this equation
18.7
27.4
64.9
80.6
107
BG
(kΩ)
V
F
R
REF
6.04
6.04
6.04
6.04
6.04
REF
(kΩ)
V
TC
. So, the next
LT3575
REF
R
FB
TC
19.1
66.5
80.6
105
?” The
28
not as
(kΩ)
REF
REF
9
3575f
FB
FB
of
:
.
,