LT3573(RevB) 查看數據表（PDF） - Linear Technology
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LT3573 Datasheet PDF : 26 Pages
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LT3573
Applications Information
ERROR AMPLIFIER—PSEUDO DC THEORY
In the Block Diagram, the RREF (R4) and RFB (R3) resistors
can be found. They are external resistors used to program
the output voltage. The LT3573 operates much the same
way as traditional current mode switchers, the major
difference being a different type of error amplifier which
derives its feedback information from the flyback pulse.
Operation is as follows: when the output switch, Q1, turns
off, its collector voltage rises above the VIN rail. The am‑
plitude of this flyback pulse, i.e., the difference between
it and VIN, is given as:
VFLBK = (VOUT + VF + ISEC • ESR) • NPS
VF = D1 forward voltage
ISEC = Transformer secondary current
ESR = Total impedance of secondary circuit
NPS = Transformer effective primary-to-secondary
turns ratio
The flyback voltage is then converted to a current by
the action of RFB and Q2. Nearly all of this current flows
through resistor RREF to form a ground-referred volt‑
age. This voltage is fed into the flyback error amplifier.
The flyback error amplifier samples this output voltage
information when the secondary side winding current is
zero. The error amplifier 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 RREF resistor to be nearly equal to the
bandgap reference voltage VBG. The relationship between
VFLBK and VBG may then be expressed as:
a
VFLBK
RFB
=
VBG
RREF
or,
VFLBK
=
VBG
RFB
RREF
1
a
a = Ratio of Q1 IC to IE, typically ≈ 0.986
VBG = Internal bandgap reference
In combination with the previous VFLBK expression yields
an expression for VOUT, in terms of the internal reference,
programming resistors, transformer turns ratio and diode
forward voltage drop:
VOUT
=
VBG
RFB
RREF
a
1
NPS
−
VF
−
ISEC
(ESR)
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 first term in the VOUT equation does not have a tem‑
perature dependence, but the diode forward drop has a
significant negative temperature coefficient. To compen-
sate for this, a positive temperature coefficient current
source is connected to the RREF pin. The current is set by
a resistor to ground connected to the TC pin. To cancel the
temperature coefficient, the following equation is used:
dVF = − RFB • 1 • dVTC or,
dT RTC NPS dT
RTC
=
−RFB
NPS
•
1
dVF / dT
•
dVTC
dT
≈
RFB
NPS
(dVF/dT) = Diode’s forward voltage temperature
coefficient
(dVTC/dT) = 2mV
VTC = 0.55V
The resistor value given by this equation should also be
verified experimentally, and adjusted if necessary to achieve
optimal regulation over temperature.
The revised output voltage is as follows:
VOUT
=
VBG
RFB
RREF
1
NPS
a
−
VF
−
VTC
RTC
•
RFB
NPS a
– ISEC
(ESR)
3573fb
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