参数资料
型号: LTC3809EMS-1#PBF
厂商: LINEAR TECHNOLOGY CORP
元件分类: 稳压器
英文描述: 1 A SWITCHING CONTROLLER, 600 kHz SWITCHING FREQ-MAX, PDSO10
封装: PLASTIC, MSOP-10
文件页数: 9/24页
文件大小: 318K
代理商: LTC3809EMS-1#PBF
38091f
LTC3809-1
17
For coincident tracking (VOUT = VX during start-up),
RTA = RA, RTB = RB
VX should always be greater than VOUT when using the
tracking function of TRACK/SS pin.
The internal current source (1
A), which is for external
soft-start, will cause a tracking error at VOUT. For example,
if a 59k resistor is chosen for RTA, the RTA current will be
about 10
A (600mV/59k). In this case, the 1A internal
current source will cause about 10% (1
A/10A 100%)
tracking error, which is about 60mV (600mV 10%)
referred to VFB. This is acceptable for most applications. If
a better tracking accuracy is required, the value of RTA
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 CSS
External Soft-Start
VIN
Internal Soft-Start
Resistor Divider
VOUT Tracking an External Voltage VX
APPLICATIO S I FOR ATIO
WU
UU
Figure 5b and 5c. Two Different Modes of Output Voltage Tracking
Auxiliary Winding Control Using the MODE Pin
The MODE pin can be used as an auxiliary feedback to
provide a means of regulating a flyback winding output.
When this pin drops below its ground-referenced 0.4V
threshold, continuous mode operation is forced.
During continuous mode, current flows 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 VIN/VOUT ratio is close to unity, the synchronous
MOSFET may not be on for a sufficient amount of time to
transfer power from the output capacitor to the auxiliary
load. Forced continuous operation will support an auxil-
iary winding as long as there is a sufficient synchronous
MOSFET duty factor. The MODE input pin removes the
requirement that power must be drawn from the trans-
former 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.
TIME
(5b) Coincident Tracking
VX
VOUT
OUTPUT
VOL
TAGE
TIME
38091 F05b,c
(5c) Ratiometric Tracking
VX
VOUT
OUTPUT
VOL
TAGE
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