LT8300
operaTion
effective quiescent current to improve light load efficiency.
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
The R
FB
resistor as depicted in the Block Diagram is the
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
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 V
IN
supply. The
amplitude of the flyback pulse, i.e., the difference between
the SW pin voltage and V
IN
supply, is given as:
V
FLBK
= (V
OUT
+ V
F
+ I
SEC
• ESR) • N
PS
V
F
= Output diode forward voltage
I
SEC
= Transformer secondary current
ESR = Total impedance of secondary circuit
N
PS
= Transformer effective primary-to-secondary
turns ratio
The flyback voltage is then converted to a current I
RFB
by
the flyback pulse sense circuit (M2 and M3). This cur-
rent I
RFB
also flows through the internal trimmed 12.23k
R
REF
resistor to generate a ground-referred voltage. The
resulting voltage 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 (I
SEC
• ESR) term in the V
FLBK
equation can be
assumed to be zero.
The bandgap reference voltage V
BG
, 1.223V, feeds to the
non-inverting input of the sample-and-hold error ampli-
fier. The relatively high gain in the overall loop causes
the voltage across R
REF
resistor to be nearly equal to the
bandgap reference voltage V
BG
. The resulting relationship
between V
FLBK
and V
BG
can be expressed as:
V
FLBK
R
• R
REF
=
V
BG
FB
or
V
V
FLBK
=
BG
• R
FB
=
I
RFB
• R
FB
R
REF
V
BG
= Bandgap reference voltage
I
RFB
= R
FB
regulation current = 100µA
Combination with the previous V
FLBK
equation yields an
equation for V
OUT
, in terms of the R
FB
resistor, transformer
turns ratio, and diode forward voltage:
V
OUT
R
FB
=
100µA •
−
V
F
N
PS
Output Temperature Coefficient
The first term in the V
OUT
equation does not have tempera-
ture dependence, but the output diode forward voltage V
F
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.
8300f
9
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