Applications Information - Linear Technology Analog Devices LT8301 Datasheet

LT8301
OPERATION
of the output voltage. The inherent minimum switch cur-
rent limit and minimum switch-off time are necessary to
guarantee the correct operation of specific applications.
As the load gets very light, the LT8301 starts to fold
back the switching frequency while keeping the mini-
mum switch current limit. So the load current is able to
decrease while still allowing minimum switch-off time for

APPLICATIONS INFORMATION

Output Voltage
The R resistor as depicted in the Block Diagram is the
FB
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 V
amplitude of the flyback pulse, i.e., the difference between
the SW pin voltage and V
V = (V + V + I
FLBK OUT F
V = Output diode forward voltage
F
I = Transformer secondary current
SEC
ESR = Total impedance of secondary circuit
N = Transformer effective primary-to-secondary
PS
turns ratio
The flyback voltage is then converted to a current I
the flyback pulse sense circuit (M2 and M3). This current
I also flows through the internal 10k R
RFB
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 ampli-
fier samples the voltage when the secondary current is
zero, the (I • ESR) term in the V
SEC
assumed to be zero.
8
supply. The
IN
supply, is given as:
IN
• ESR) • N
SEC PS
resistor to
REF
equation can be
FLBK
For more information
the sample-and-hold error amplifier. Meanwhile, the part
switches between sleep mode and active mode, thereby
reducing the effective quiescent current to improve light
load efficiency. In this condition, the LT8301 operates in
low ripple Burst Mode. The 10kHz (typ) minimum switch-
ing frequency determines how often the output voltage is
sampled and also the minimum load requirement.
An internal trimmed reference voltage,V
to the non-inverting input of the sample-and-hold error
amplifier. The relatively high gain in the overall loop
causes the voltage across R
to V . The resulting relationship between V
IREF
V can be expressed as:
IREF
V
⎛ ⎞
FLBK
• R
⎝ ⎜ ⎠ ⎟
R
FB
or
V
⎛
IREF
V
=
FLBK
⎝ ⎜
R
V = Internal trimmed reference voltage
IREF
I = R regulation current = 100µA
RFB FB
Combination with the previous V
equation for V
former turns ratio, and diode forward voltage:
V = 100µA •
OUT
by
RFB
Output Temperature Coefficient
The first term in the V
perature dependence, but the output diode forward volt-
age V has a significant negative temperature coefficient
F
(–1mV/°C to –2mV/°C). Such a negative temperature coef-
ficient produces approximately 200mV to 300mV voltage
variation on the output voltage across temperature.
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resistor to be nearly equal
REF
= V
REF IREF
⎞
• R = I • R
FB RFB FB
⎠ ⎟
REF
equation yields an
FLBK
, in terms of the R
OUT FB
R
⎛ ⎞
FB
− V
F
⎝ ⎜ ⎠ ⎟
N
PS
equation does not have tem-
OUT
1.0V, feeds
IREF
and
FLBK
resistor, trans-
Rev. A