LT8300
operaTion
effectivequiescentcurrenttoimprovelightloadefficiency.
In this condition, the LT8300 operates in low ripple Burst
Mode. The typical 7.5kHz minimum switching frequency
determines how often the output voltage is sampled and
also the minimum load requirement.
applicaTions inForMaTion
Output Voltage
bandgap reference voltage V . The resulting relationship
BG
between V
and V can be expressed as:
FLBK
BG
The R resistor as depicted in the Block Diagram is the
FB
only external resistor used to program the output voltage.
The LT8300 operates similar to traditional current mode
switchers, except in the use of a unique flyback pulse
sensecircuitandasample-and-holderroramplifier, which
sample and therefore regulate the isolated output voltage
from the flyback pulse.
V
FLBK
• R
= V
BG
REF
R
FB
or
V
BG
V
=
• R = I
• R
RFB FB
FLBK
FB
R
REF
Operation is as follows: when the power switch M1 turns
V
= Bandgap reference voltage
BG
off, the SW pin voltage rises above the V supply. The
IN
amplitude of the flyback pulse, i.e., the difference between
I
= R regulation current = 100µA
FB
RFB
the SW pin voltage and V supply, is given as:
IN
Combination with the previous V
equation yields an
FLBK
V
FLBK
= (V
+ V + I
• ESR) • N
SEC PS
equationforV , intermsoftheR resistor, transformer
OUT
F
OUT
FB
turns ratio, and diode forward voltage:
V = Output diode forward voltage
F
RFB
I
= Transformer secondary current
SEC
VOUT = 100µA •
− V
F
N
PS
ESR = Total impedance of secondary circuit
N
= Transformer effective primary-to-secondary
turns ratio
Output Temperature Coefficient
ThefirsttermintheV equationdoesnothavetempera-
PS
OUT
The flyback voltage is then converted to a current I
the flyback pulse sense circuit (M2 and M3). This cur-
rent I
REF
resultingvoltagefeedstotheinvertinginputofthesample-
and-hold error amplifier. Since the sample-and-hold error
amplifier samples the voltage when the secondary current
is zero, the (I • ESR) term in the V
assumed to be zero.
by
ture dependence, but the output diode forward voltage V
RFB
F
hasasignificantnegativetemperaturecoefficient(–1mV/°C
to–2mV/°C). Suchanegativetemperaturecoefficientpro-
duces approximately 200mV to 300mV voltage variation
on the output voltage across temperature.
also flows through the internal trimmed 12.23k
RFB
R
resistor to generate a ground-referred voltage. The
Forhighervoltageoutputs,suchas12Vand24V,theoutput
diodetemperaturecoefficienthasanegligibleeffectonthe
output voltage regulation. For lower voltage outputs, such
as 3.3V and 5V, however, the output diode temperature
coefficientdoescountforanextra2%to5%outputvoltage
regulation. For customers requiring tight output voltage
regulation across temperature, please refer to other LTC
parts with integratedtemperaturecompensation features.
equation can be
SEC
FLBK
The bandgap reference voltage V , 1.223V, feeds to the
BG
non-inverting input of the sample-and-hold error ampli-
fier. The relatively high gain in the overall loop causes
the voltage across R resistor to be nearly equal to the
REF
8300f
9
Linear [ Linear ]
