lm5021na-2 National Semiconductor Corporation, lm5021na-2 Datasheet - Page 9

• Manufacturer Part Number: lm5021na-2
• Description: Ac-dc Current Mode Pwm Controller
• Manufacturer: National Semiconductor Corporation

Detailed Operating Description
frequency and the operational output frequency are the
same. To set a desired output switching frequency (Fsw), the
RT resistor can be calculated from:
LM5021-1:
LM5021-2:
The LM5021 can also be synchronized to an external clock.
The external clock must have a higher frequency than the
free running oscillator frequency set by the RT resistor. The
clock signal should be capacitively coupled into the RT pin
with a 100pF capacitor. A peak voltage level greater than 3.8
Volts at the RT pin is required for detection of the sync pulse.
The dc voltage across the RT resistor is internally regulated
at 2 volts. Therefore, the ac pulse superimposed on the RT
resistor must have 1.8V or greater amplitude to successfully
synchronize the oscillator. The sync pulse width should be
set between 15ns to 150ns by the external components. The
RT resistor is always required, whether the oscillator is free
running or externally synchronized. The RT resistor should
be located very close to the device and connected directly to
the pins of the LM5021 (RT and GND).
GATE DRIVER and MAX DUTY CYCLE LIMIT
The LM5021 provides a gate driver (OUT), which can source
peak current of 0.3A and sink 0.7A. The LM5021 is available
in two duty-cycle limit options. The maximum output duty-
cycle is typically 80% for the LM5021-1 option, and precisely
equal to 50% for the LM5021-2 option. The maximum duty
cycle function for the LM5021-2 is accomplished with an
internal toggle flip-flop to ensure an accurate duty cycle limit.
The internal oscillator frequency of the LM5021-2 is there-
fore twice the switching frequency of the PWM controller
(OUT pin).
The 80% maximum duty-cycle function for the LM5021-1 is
determined by the internal oscillator. For the LM5021-1 the
internal oscillator frequency and the switching frequency of
the PWM controller are the same.
SOFT-START
The soft-start feature allows the power converter to gradually
reach the initial steady state operating point, thus reducing
start-up stresses and current surges. An internal 22 µA
current source charges an external capacitor connected to
the SS pin. The capacitor voltage will ramp up slowly, limiting
the COMP pin voltage and the duty cycle of the output
pulses. The soft-start capacitor is also used to generate the
hiccup mode delay time when the output of the switching
power supply is continuously overloaded.
HICCUP MODE OVERLOAD CURRENT LIMITING
Hiccup mode is a method of protecting the power supply
from over-heating and damage during an extended overload
condition. When the output fault is removed the power sup-
ply will automatically restart.
(Continued)
9
Figure 3, Figure 4 and Figure 5 illustrate the equivalent
circuit of the hiccup mode for LM5021 and the relevant
waveforms. During start-up and in normal operation, the
external soft-start capacitor Css is pulled up by a current
source that delivers 22 µA to the SS pin capacitor. In normal
operation, the soft-start capacitor continues to charge and
eventually reaches the saturation voltage of the current
source (V
COMP pin voltage follows the SS capacitor voltage and
gradually increases the peak current delivered by the power
supply. When the output of the switching power supply
reaches the desired voltage, the voltage feedback amplifier
takes control of the COMP signal (via the opto-coupler). In
normal operation the COMP level is held at an intermediate
voltage between 1.25V and 2.75V controlled by the voltage
regulation loop. When the COMP pin voltage is below 1.25V,
the duty-cycle is zero. When the COMP level is above 2.75V,
the duty cycle will be limited by the 0.5V threshold of cycle-
by-cycle current limit comparator.
If the output of the power supply is overloaded, the voltage
regulation loop demands more current by increasing the
COMP pin control voltage. When the COMP pin exceeds the
over voltage detection threshold (V
the SS capacitor Css will be discharged by a 10 µA overload
detection timer current source, I
above V
to the Hiccup mode threshold (V
controller enters the hiccup mode. The OUT pin is then
latched low and the SS capacitor discharge current source is
reduced from 10 µA to 0.25 µA, the dead-time current
source, I
reaches the Restart threshold (V
a new start-up sequence commences with 22 µA current
source charging the capacitor C
the SS capacitor from the Hiccup threshold to the Restart
threshold provides an extended off time that reduces the
overheating of components including diodes and MOSFETs
due to the continuous overload. The off time during the
hiccup mode can be calculated from the following equation:
Example:
Toff = 808 ms, assuming the C
Short duration intermittent overloads will not trigger the hic-
cup mode. The overload duration required to trigger the
hiccup response is set by the capacitor C
discharge current source and voltage difference between the
saturation level of the SS pin and the Hiccup mode thresh-
old. Figure 5 shows the waveform of SS pin with a short
duration overload condition. The overload time required to
enter the hiccup mode can be calculated from the following
equation:
Example:
Toverload = 2.82 ms, assuming the C
0.047 µF
OVLD
DTCS
SS_OCV
. The SS pin voltage is slowly reduced until it
long enough for the SS capacitor to discharge
, nominally 5.2V). During start-up the
SS
RST
SS
capacitor value is 0.047 µF
OVCS
HIC
. The slow discharge of
, nominally 0.3V). Then
OVLD
, nominally 4.6V), the
SS
. If COMP remains
, nominally 4.6V),
capacitor value is
SS
, the 10 µA
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