LT8301MPS5-TRPBF 查看數據表（PDF） - Linear Technology
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LT8301MPS5-TRPBF
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LT8301MPS5-TRPBF Datasheet PDF : 24 Pages
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LT8301
APPLICATIONS INFORMATION
Output Voltage
The RFB resistor as depicted in the Block Diagram is the
only external resistor used to program the output voltage.
The LT8301 operates similar to traditional current mode
switchers, except in the use of a unique flyback pulse
sense circuit and a sample-and-hold error amplifier, which
sample and therefore regulate the isolated output voltage
from the flyback pulse.
Operation is as follows: when the power switch M1 turns
off, the SW pin voltage rises above the VIN supply. The
amplitude of the flyback pulse, i.e., the difference between
the SW pin voltage and VIN supply, is given as:
VFLBK = (VOUT + VF + ISEC • ESR) • NPS
VF = Output diode 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 IRFB by
the flyback pulse sense circuit (M2 and M3). This current
IRFB also flows through the internal 10k RREF resistor to
generate a ground-referred voltage. The resulting volt-
age feeds to the inverting input of the sample-and-hold
error amplifier. Since the sample-and-hold error amplifier
samples the voltage when the secondary current is zero,
the (ISEC • ESR) term in the VFLBK equation can be as-
sumed to be zero.
An internal trimmed reference voltage,VIREF 1.0V, feeds
to the non-inverting input of the sample-and-hold error
amplifier. The relatively high gain in the overall loop causes
the voltage across RREF resistor to be nearly equal to VIREF.
The resulting relationship between VFLBK and VIREF can
be expressed as:
VFLBK
RFB
• RREF
=
VIREF
or
VFLBK
=
VIREF
RREF
• RFB
= IRFB
• RFB
VIREF = Internal trimmed reference voltage
IRFB = RFB regulation current = 100µA
Combination with the previous VFLBK equation yields an
equation for VOUT, in terms of the RFB resistor, transformer
turns ratio, and diode forward voltage:
VOUT
=
100µA
•
RFB
NPS
−
VF
Output Temperature Coefficient
The first term in the VOUT equation does not have tempera-
ture dependence, but the output diode forward voltage VF
has a significant negative temperature coefficient (–1mV/°C
to –2mV/°C). Such a negative temperature coefficient pro-
duces approximately 200mV to 300mV voltage variation
on the output voltage across temperature.
For higher voltage outputs, such as 12V and 24V, the output
diode temperature coefficient has a negligible effect on the
output voltage regulation. For lower voltage outputs, such
as 3.3V and 5V, however, the output diode temperature
coefficient does count for an extra 2% to 5% output voltage
regulation. For customers requiring tight output voltage
regulation across temperature, please refer to other LTC
parts with integrated temperature compensation features.
8301f
For more information www.linear.com/LT8301
9
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