Datasheet

LTC3809-1
17
38091fc
For coincident tracking (V
OUT
= V
X
during start-up),
R
TA
= R
A
, R
TB
= R
B
V
X
should always be greater than V
OUT
when using the
tracking function of TRACK/SS pin.
The internal current source (1μA), which is for external
soft-start, will cause a tracking error at V
OUT
. For example,
if a 59k resistor is chosen for R
TA
, the R
TA
current will be
about 10μA (600mV/59k). In this case, the 1μA internal
current source will cause about 10% (1μA/10μA • 100%)
tracking error, which is about 60mV (600mV • 10%)
referred to V
FB
. This is acceptable for most applications.
If a better tracking accuracy is required, the value of R
TA
should be reduced.
Table 1 summarizes the different states in which the
TRACK/SS can be used.
Table 1. The States of the TRACK/SS Pin
TRACK/SS Pin FREQUENCY
Capacitor C
SS
External Soft-Start
V
IN
Internal Soft-Start
Resistor Divider V
OUT
Tracking an External Voltage V
X
APPLICATIONS INFORMATION
Auxiliary Winding Control Using the MODE Pin
The MODE pin can be used as an auxiliary feedback to
provide a means of regulating a fl yback winding output.
When this pin drops below its ground-referenced 0.4V
threshold, continuous mode operation is forced.
During continuous mode, current fl ows continuously in
the transformer primary side. The auxiliary winding draws
current only when the bottom synchronous N-channel
MOSFET is on. When primary load currents are low and/
or the V
IN
/V
OUT
ratio is close to unity, the synchronous
MOSFET may not be on for a suffi cient amount of time to
transfer power from the output capacitor to the auxiliary
load. Forced continuous operation will support an auxiliary
winding as long as there is a suffi cient synchronous
MOSFET duty factor. The MODE input pin removes
the requirement that power must be drawn from the
transformer primary side in order to extract power from
the auxiliary winding. With the loop in continuous mode,
the auxiliary output may nominally be loaded without
regard to the primary output load.
Figure 5b and 5c. Two Different Modes of Output Voltage Tracking
TIME
(5b) Coincident Tracking
V
X
V
OUT
OUTPUT VOLTAGE
TIME
38091 F05b,c
(5c) Ratiometric Tracking
V
X
V
OUT
OUTPUT VOLTAGE