LT3575EFE#TRPBF Linear Technology, LT3575EFE#TRPBF Datasheet - Page 8

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

Specifications of LT3575EFE#TRPBF

• 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
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Duty Cycle: -

LT3575
ERROR AMPLIFIER—PSEUDO DC THEORY
In the Block Diagram, the R
can be found. They are external resistors used to program
the output voltage. The LT3575 operates much the same way
as traditional current mode switchers, the major difference
being a different type of error amplifi er which derives its
feedback information from the fl yback pulse.
Operation is as follows: when the output switch, Q1,
turns off, its collector voltage rises above the V
amplitude of this fl yback pulse, i.e., the difference between
it and V
The fl yback voltage is then converted to a current by
the action of R
through resistor R
This voltage is fed into the fl yback error amplifi er. The
fl yback error amplifi er samples this output voltage
information when the secondary side winding current is
zero. The error amplifi er uses a bandgap voltage, 1.23V,
as the reference voltage.
The relatively high gain in the overall loop will then cause
the voltage at the R
bandgap reference voltage V
V
APPLICATIONS INFORMATION
8
FLBK
V
V
I
ESR = Total impedance of secondary circuit
N
turns ratio
V
V
SEC
FLBK
F
BG
PS
FLBK
= Ratio of Q1 I
= D1 forward voltage
and V
V
= Internal bandgap reference
= Transformer secondary current
IN
R
= Transformer effective primary-to-secondary
FLBK
= (V
FB
, is given as:
BG
V
OUT
BG
FB
may then be expressed as:
+ V
R
and Q2. Nearly all of this current fl ows
R
V
REF
R
REF
BG
C
REF
REF
FB
F
to I
+ I
to form a ground-referred voltage.
resistor to be nearly equal to the
E
SEC
or
, typically 0.986
REF
1
,
BG
• ESR) • N
(R4) and R
. The relationship between
PS
FB
(R3) resistors
IN
rail. The
In combination with the previous V
an expression for V
programming resistors, transformer turns ratio and diode
forward voltage drop:
Additionally, it includes the effect of nonzero secondary
output impedance (ESR). This term can be assumed to
be zero in boundary control mode. More details will be
discussed in the next section.
Temperature Compensation
The fi rst term in the V
perature dependence, but the diode forward drop has a
signifi cant negative temperature coeffi cient. To compen-
sate for this, a positive temperature coeffi cient current
source is connected to the R
a resistor to ground connected to the TC pin. To cancel the
temperature coeffi cient, the following equation is used:
The resistor value given by this equation should also be
verifi ed experimentally, and adjusted if necessary to achieve
optimal regulation overtemperature.
The revised output voltage is as follows:
( V
coeffi cient
( V
V
R
V
V
TC
OUT
OUT
V
TC
T
F
TC
F
= 0.55V
/
/ T) = 2mV
T
N
) = Diode’s forward voltage temperature
V
V
R
R
R
PS
BG
BG
FB
TC
FB
R
V
•
TC
TC
R
R
R
•
R
REF
OUT
REF
FB
V
FB
N
F F
•
OUT
PS
1
1
, in terms of the internal reference,
/
N
R
PS
•
T
equation does not have a tem-
FB
N
PS
•
N
REF
1
1
V
PS
–
T
TC
V
pin. The current is set by
I
T
TC
SEC
FLBK
or
V
V
(
F
ESR
N
R
F
,
expression yields
PS
FB
I
SEC
)
(
ES
R R )
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