APPLICATION NOTE
UNDER-VOLTAGE LOCKOUT
U-100A
The UVLO circuit insures that is adequate to make
the fully operational before enabling the
output stage. Figure 5 shows that the UVLO turn-on and
turn-off thresholds are fixed internally at 16V and 10V re-
spectively. The 6V hysteresis prevents oscillations
during power sequencing. Figure 6 shows supply current
requirements. Start-up current is less than 1 mA for effi-
cient bootstrapping from the rectified input of an off-line
converter, as illustrated by Figure 6. During normal circuit
operation,
is developed from auxiliary winding
with and At start-up, however, must be
charged to 16V through With a start-up current of 1
mA, can be as large as 100 and still charge GIN
when = 90V RMS (low line). Power dissipation in
would then be less than 350 mW even under high line
= 130V RMS) conditions.
During UVLO; the output driver is in a low state. While it
doesn’t exhibit the same saturation characteristics as nor-
mal operation, it can easily sink 1 milliamp, enough to in-
sure the MOSFET is held off.
Figure 6. During Under-Voltage Lockout, the output
driver is biased to sink minor amounts of
current.
OSCILLATOR
The UC3842 oscillator is programmed as shown in Figure
8. Timing capacitor CT is charged from (5V) through
the timing resistor
and discharged by an internal cur-
rent source.
The first step in selecting the oscillator components is to
determine the required circuit deadtime. Once obtained,
Figure 9 is used to pinpoint the nearest standard value of
CT for a given deadtime. Next, the appropriate value is
interpolated using the parameters for and oscillator
frequency. Figure 10 illustrates the combinations
versus oscillator frequency. The timing resistor can be cal-
culated from the following formula.
(kHz) = 1.72 / (k) ×
The UC3844 and UC3845 have an internal divide-by-two
flip-flop driven by the oscillator for a 50% maximum duty
cycle. Therefore, their oscillators must be set to run at
twice the desired power supply switching frequency. The
UC3842 and UC3843 oscillator runs AT the switching fre-
quency. Each oscillator of the family can
be used to a maximum of 500 kHz.
Figure 7. Providing Power to the
3-56
0019-8
图
6
:在欠压闭锁期间,对输出驱动器施加了偏压,以吸收
较少量的电流。
欠压闭锁
UVLO
电路用于确保
V
CC
足以在启用输出级之前使
UC3842/3/4/5
全面运作。如图
5
所示,
UVLO
接通和关断门限分别在内部固
定于
16V
和
10V
。
6V
迟滞用于防止在电源排序期间发生
V
CC
振
荡。图
6
示出了电源电流要求。由于能够从一个隔离式转换器
的整流输入实现高效的自举,因此启动电流小于
1mA
(如图
6
所示)。在正常电路操作期间,
V
CC
利用
D1
和
C
IN
从辅助绕组
WAUX
产生。然而,在启动时,
CIN
必须通过
RIN
充电至
16V
。
由于启动电流为
1mA
,因此
RIN
的阻值最大可至
100k
Ω,而
且在
V
AC
= 90VRMS
(低线路输入电压)时仍然对
GIN
进行充
电。
RIN
中的功耗于是将小于
350mW
,即使在高线路输入电压
(V
AC
= 130VRMS)
条件下也不例外。
在
UVLO
期间;输出驱动器处于低电平状态。尽管它所呈现的
饱和特性与正常操作时有所不同,但它仍然能够很容易地吸收
1mA
的电流,这足以确保
MOSFET
保持关断状态。
振荡器
UC3842
振荡器的设置如图
8
所示。定时电容器
CT
通过定时电
阻器
RT
从
V
REF
(5V)
来充电,并由一个内部电流源进行放电。
在选择振荡器元件的过程中,第一步是确定所需的电路死区时
间。一旦确定了死区时间,则使用图
9
来准确地找出与给定死
区时间相对应的最接近的
C
T
标准值。接着,采用
C
T
和振荡器频
率的参数来插入合适的
R
T
值。图
10
示出了
R
T
/C
T
组合与振荡器
频率的关系曲线。定时电阻器的阻值可以由下式来计算。
Fosc (kHz) = 1.72 / (R
T
(k)
×
C
T
(
μ
f))
UC3844
和
UC3845
具有一个由振荡器来驱动的内部二分频触
发器,以提供一个
50%
的最大占空比。因此,必须将其振荡
器的运行频率设定为期望的电源开关频率的两倍。
UC3842
和
UC3843
振荡器在开关频率条件下运行。
UC3842/3/4/5
系列的
每款振荡器都可在高至
500kHz
(最大值)的频率下使用。
APPLICATION NOTE
UNDER-VOLTAGE LOCKOUT
U-100A
The UVLO circuit insures that is adequate to make
the fully operational before enabling the
output stage. Figure 5 shows that the UVLO turn-on and
turn-off thresholds are fixed internally at 16V and 10V re-
spectively. The 6V hysteresis prevents oscillations
during power sequencing. Figure 6 shows supply current
requirements. Start-up current is less than 1 mA for effi-
cient bootstrapping from the rectified input of an off-line
converter, as illustrated by Figure 6. During normal circuit
operation, is developed from auxiliary winding
with and At start-up, however, must be
charged to 16V through With a start-up current of 1
mA, can be as large as 100 and still charge GIN
when = 90V RMS (low line). Power dissipation in
would then be less than 350 mW even under high line
= 130V RMS) conditions.
During UVLO; the output driver is in a low state. While it
doesn’t exhibit the same saturation characteristics as nor-
mal operation, it can easily sink 1 milliamp, enough to in-
sure the MOSFET is held off.
Figure 6. During Under-Voltage Lockout, the output
driver is biased to sink minor amounts of
current.
OSCILLATOR
The UC3842 oscillator is programmed as shown in Figure
8. Timing capacitor CT is charged from (5V) through
the timing resistor and discharged by an internal cur-
rent source.
The first step in selecting the oscillator components is to
determine the required circuit deadtime. Once obtained,
Figure 9 is used to pinpoint the nearest standard value of
CT for a given deadtime. Next, the appropriate value is
interpolated using the parameters for and oscillator
frequency. Figure 10 illustrates the combinations
versus oscillator frequency. The timing resistor can be cal-
culated from the following formula.
(kHz) = 1.72 / (k) ×
The UC3844 and UC3845 have an internal divide-by-two
flip-flop driven by the oscillator for a 50% maximum duty
cycle. Therefore, their oscillators must be set to run at
twice the desired power supply switching frequency. The
UC3842 and UC3843 oscillator runs AT the switching fre-
quency. Each oscillator of the family can
be used to a maximum of 500 kHz.
Figure 7. Providing Power to the
3-56
0019-8