參數(shù)資料
型號(hào)： AS2845
英文描述： Current Mode Controller
中文描述： 電流模式控制器
文件頁(yè)數(shù)： 11/20頁(yè)
文件大?。?/td> 94K
代理商： AS2845
ASTEC Semiconductor
Current Mode Controller
AS2842/3/4/5
47
(C) is charged via resistor (R) from the rectified
AC line. When the voltage on the capacitor (V
CC
)
reaches the upper UVLO threshold, the IC (and
hence, the power supply) turns on and the volt-
age on C begins to quickly discharge due to the
increased operating current. During this time, the
auxiliary winding begins to supply the current
necessary to run the IC. The capacitor must be
sufficiently large to maintain a voltage greater
than the lower UVLO threshold during start up.
The value of R must be selected to provide
greater than 1 mA of current at the minimum DC
bus voltage (R < VDCmin/1 mA).
The UVLO feature of the AS2842 has significant
advantages over standard 2842 devices. First,
the UVLO thresholds are based on a tempera-
ture compensated band gap reference rather
than conventional zeners. Second, the UVLO
disables the output at power down, offering addi-
tional protection in cases where V
REG
is heavily
decoupled. The UVLO on some 2842 devices
shuts down the 5 volt regulator only, which
results in eventual power down of the output only
after the 5 volt rail collapses. This can lead to
unwanted stresses on the switching devices dur-
ing power down. The AS2842 has two separate
comparators which monitor both V
CC
and V
REF
and hold the output low if either are not within
specification.
The AS2842 family offers two different UVLO
options. The AS2842/4 has UVLO thresholds of
16 volts (on) and 10 volts (off). The AS2843/5 has
UVLO levels of 8.4 volts (on) and 7.6 volts (off).
1.2 Reference (V
REG
and V
FB
)
The AS2842 effectively has two precise band
gap based temperature compensated voltage
references. Most obvious is the V
REG
pin (pin 8)
which is the output of a series pass regulator.
This 5.0 V output is normally used to provide
charging current to the oscillator’s timing
capacitor (Section 1.3). In addition, there is a
trimmed internal 2.5 V reference which is con-
nected to the non-inverting (+) input of the error
amplifier. The tolerance of the internal reference
is
±
0.5% over the full specified temperature range,
and
±
1% for V
REG.
The reference section of the AS2842 is greatly
improved over the standard 2842 in a number of
ways. For example, in a closed loop system, the
voltage at the error amplifier’s inverting input
(V
FB
, pin 1) is forced by the loop to match the
voltage at the non-inverting input. Thus, V
FB
is
the voltage which sets the accuracy of the entire
system. The 2.5 V reference of the AS2842 is
tightly trimmed for precision at V
FB
, including
errors caused by the op amp, and is specified
over temperature. This method of trim provides a
precise reference voltage for the error amplifier
while maintaining the original 5 V regulator speci-
fications. In addition, force/sense (Kelvin) bond-
ing to the package pin is utilized to further im-
prove the 5 V load regulation. Standard 2842’s,
on the other hand, specify tight regulation for the
5 V output only and rate it over line, load and
temperature. The voltage at V
FB
, which is of
critical importance, is loosely specified and only
at 25
°
C.
The reference section, in addition to providing a
precise DC reference voltage, also powers most
of the IC’s internal circuitry. Switching noise,
therefore, can be internally coupled onto the
reference. With this in mind, all of the logic within
the AS2842 was designed with ECL type circuitry
which generates less switching noise because it
runs at essentially constant current regardless of
logic state. This, together with improved AC
noise rejection, results in substantially less switch-
ing noise on the 5 V output.
The reference output is short circuit protected
and can safely deliver more than 20 mA to power
external circuitry.
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