LT3837
11
3837fd
OPERATION
Minimum Output Switch On-Time (t
ON(MIN)
)
The LT3837 affects output voltage regulation via flyback
pulse action. If the output switch is not turned on, there
is no flyback pulse and output voltage information is
not available. This causes irregular loop response and
start-up/latch-up problems. The solution is to require the
primary switch to be on for an absolute minimum time
per each oscillator cycle. If the output load is less than
that developed under these conditions, forced continuous
operation normally occurs. See the Applications Informa-
tion section for further details.
Enable Delay (ENDLY)
The flyback pulse appears when the primary side switch
shuts off. However, it takes a finite time until the transformer
primary side voltage waveform represents the output
voltage. This is partly due to rise time on the primary
side MOSFET drain node but, more importantly, is due to
transformer leakage inductance. The latter causes a voltage
spike on the primary side, not directly related to output
voltage. Some time is also required for internal settling
of the feedback amplifier circuitry. In order to maintain
immunity to these phenomena, a fixed delay is introduced
between the switch turn-off command and the enabling of
the feedback amplifier. This is termed “enable delay.” In
certain cases where the leakage spike is not sufficiently
settled by the end of the enable delay period, regulation
error may result. See the Applications Information section
for further details.
Collapse Detect
Once the feedback amplifier is enabled, some mechanism
is then required to disable it. This is accomplished by a
collapse detect comparator, which compares the flyback
voltage (FB referred) to a fixed reference, nominally 80%
of V
FB
. When the flyback waveform drops below this level,
the feedback amplifier is disabled.
Minimum Enable Time
The feedback amplifier, once enabled, stays enabled for
a fixed minimum time period termed “minimum enable
time.” This prevents lockup, especially when the output
voltage is abnormally low; e.g., during start-up. The mini-
mum enable time period ensures that the V
C
node is able
to pump up and increase the current mode trip point to
the level where the collapse detect system exhibits proper
operation. This time is internally set.
Effects of Variable Enable Period
The feedback amplifier is enabled during only a portion of
the cycle time. This can vary from the fixed minimum enable
time described to a maximum of roughly the “off” switch
time minus the enable delay time. Certain parameters of
flyback amp behavior are directly affected by the variable
enable period. These include effective transconductance
and V
C
node slew rate.
Load Compensation Theory
The LT3837 uses the flyback pulse to obtain information
about the isolated output voltage. An error source is
caused by transformer secondary current flow through the
synchronous MOSFET R
DS(ON)
and real life nonzero imped-
ances of the transformer secondary and output capacitor.
This was represented previously by the expression “I
SEC
• (ESR + R
DS(ON)
).” However, it is generally more useful
to convert this expression to effective output impedance.
Because the secondary current only flows during the off
portion of the duty cycle (DC), the effective output imped-
ance equals the lumped secondary impedance divided by
OFF time DC.
Since the OFF time duty cycle is equal to 1 – DC then:
R
S(OUT)
=
ESR+R
DS(ON)
1–DC
where:
R
S(OUT)
= effective supply output impedance
DC = duty cycle
R
DS(ON)
and ESR are as defined previously
This impedance error may be judged acceptable in less
critical applications, or if the output load current remains
relatively constant. In these cases the external FB resistive
divider is adjusted to compensate for nominal expected
error. In more demanding applications, output impedance
error is minimized by the use of the load compensation
function.
