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PDF LM5021 Data sheet ( Hoja de datos )
| Número de pieza | LM5021 | |
| Descripción | AC-DC Current Mode PWM Controller | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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May 2005
www.DataSheet4U.com
LM5021
AC-DC Current Mode PWM Controller
General Description
The LM5021 off-line pulse width modulation (PWM) control-
ler contains all of the features needed to implement highly
efficient off-line single-ended flyback and forward power con-
verters using current-mode control. The LM5021 features
include an ultra-low (25 µA) start-up current, which mini-
mizes power losses in the high voltage start-up network. A
skip cycle mode reduces power consumption with light loads
for energy conserving applications (ENERGY STAR®,
CECP, etc.). Additional features include under-voltage lock-
out, cycle-by-cycle current limit, hiccup mode overload pro-
tection, slope compensation, soft-start and oscillator syn-
chronization capability. This high performance 8-pin IC has
total propagation delays less than 100nS and a 1MHz ca-
pable oscillator that is programmed with a single resistor.
Features
n Ultra Low Start-up Current (25 µA maximum)
n Current Mode Control
n Skip Cycle Mode for Low Standby Power
n Single Resistor Programmable Oscillator
n Synchronizable Oscillator
n Adjustable Soft-start
n Integrated 0.7A Peak Gate Driver
n Direct Opto-Coupler Interface
n Maximum Duty Cycle Limiting (80% for LM5021-1 or
50% for LM5021-2)
n Slope Compensation for (LM5021-1 Only)
n Under Voltage Lockout (UVLO) with Hysteresis
n Cycle-by-Cycle Over-Current Protection
n Hiccup Mode for Continuous Overload Protection
n Leading Edge Blanking of Current Sense Signal
n Packages: MSOP-8 or MDIP-8
Simplified Application Diagram
© 2005 National Semiconductor Corporation DS201442
20144201
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1 page  
Simplified Block Diagram
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FIGURE 1.
5
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5 Page 
Detailed Operating Description (Continued)
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FIGURE 5. Waveform at SS and COMP Pin due to Brief Overload
20144221
SKIP CYCLE OPERATION
During light load conditions, the efficiency of the switching
power supply typically drops as the losses associated with
switching and operating bias currents of the converter be-
come a significant percentage of the power delivered to the
load. The largest component of the power loss is the switch-
ing loss associated with the gate driver and external MOS-
FET gate charge. Each PWM cycle consumes a finite amout
of energy as the MOSFET is turned on and then turned off.
These switching losses are proportional to the frequency of
operation. The Skip Cycle function integrated within the
LM5021 controller reduces the average switching frequency
to reduce switching losses and improve efficiency during
light load conditions.
When a light load condition occurs, the COMP pin voltage is
reduced by the voltage feedback loop to reduce the peak
current delivered by the controller. Referring to Figure 6, the
PWM comparator input tracks the COMP pin voltage through
a 1.25V level shift circuit and a 3:1 resistor divider. As the
COMP pin voltage falls, the input to the PWM comparator
falls proportionately. When the PWM comparator input falls
to 125mV, the Skip Cycle comparator detects the light load
condition and disables output pulses from the controller. The
controller continues to skip switching cycles until the power
supply output falls and the COMP pin voltage increases to
demand more output current. The number of cycles skipped
will depend on the load and the response time of the fre-
quency compensation network. Eventually the COMP volt-
age will increase when the voltage loop requires more cur-
rent to sustain the regulated output voltage. When the PWM
comparator input exceeds 130mV (5mV hysteresis), normal
fixed frequency switching resumes. Typical power supply
designs will produce a short burst of output pulses followed
by a long skip cycle interval. The average switching fre-
quency in the Skip Cycle mode can be a small fraction of the
normal operating frequency of the power supply.
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| Páginas | Total 15 Páginas | |
| PDF Descargar | [ Datasheet LM5021.PDF ] | |
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