lm5020sdx-2 National Semiconductor Corporation, lm5020sdx-2 Datasheet - Page 8

• Manufacturer Part Number: lm5020sdx-2
• Description: 100v Current Mode Pwm Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
Detailed Operating Description
The LM5020 High Voltage PWM controller contains all of the
features needed to implement single ended primary power
converter topologies. The LM5020 includes a high-voltage
startup regulator that operates over a wide input range to
100V. The PWM controller is designed for high speed capa-
bility including an oscillator frequency range to 1MHz and
total propagation delays less than 100ns. Additional features
include an error amplifier, precision reference, line under-
voltage lockout, cycle-by-cycle current limit, slope compen-
sation, softstart, oscillator sync capability and thermal shut-
down. The functional block diagram of the LM5020 is shown
in Figure 1. The LM5020 is designed for current-mode con-
trol power converters, which require a single drive output,
such as Flyback and Forward topologies. The LM5020 pro-
vides all of the advantages of current-mode control including
line feed-forward, cycle-by-cycle current limiting and simpli-
fied loop compensation .
High Voltage Start-Up Regulator
The LM5020 contains an internal high voltage startup regu-
lator, that allows the input pin (Vin) to be connected directly
to line voltages as high as 100V. The regulator output is
internally current limited to 15mA. When power is applied,
the regulator is enabled and sources current into an external
capacitor connected to the V
pacitance range for the Vcc regulator is 0.1µF to 100µF.
When the voltage on the V
level of 7.7V, the controller output is enabled. The controller
will remain enabled until V
In typical applications, a transformer auxiliary winding is
connected through a diode to the V
should raise the V
startup regulator. Powering V
improves conversion efficiency while reducing the power
dissipated in the controller. The external V
be selected such that the capacitor maintains the Vcc volt-
age greater than the V
the initial start-up. During a fault condition when the con-
verter auxiliary winding is inactive, external current draw on
the V
in the start-up regulator does not exceed the maximum
power dissipation capability of the controller.
An external start-up or other bias rail can be used instead of
the internal start-up regulator by connecting the V
Vin pins together and feeding the external bias voltage (8-
15V) to the two pins.
Line Under Voltage Detector
The LM5020 contains a line Under Voltage Lock Out (UVLO)
circuit. An external set-point voltage divider from Vin to GND
sets the operational range of the converter. The resistor
divider must be designed such that the voltage at the UVLO
pin is greater than 1.25V when Vin is in the desired operating
range. If the under voltage threshold is not met, all functions
of the controller are disabled and the controller remains in a
low power standby state.
CC
line should be limited such that the power dissipated
CC
voltage above 8V to shut off the internal
CC
UVLO falling threshold (6V) during
CC
CC
CC
falls below 6V.
CC
pin. The recommended ca-
pin reaches the regulation
from an auxiliary winding
CC
pin. This winding
CC
capacitor must
CC
and the
8
UVLO hysteresis is accomplished with an internal 20µA
current source that is switched on or off into the impedance
of the set-point divider. When the UVLO threshold is ex-
ceeded, the current source is activated to instantly raise the
voltage at the UVLO pin. When the UVLO pin voltage falls
below the 1.25V threshold the current source is turned off,
causing the voltage at the UVLO pin to fall. The UVLO pin
can also be used to implement a remote enable / disable
function. If an external transistor pulls the UVLO pin below
the 1.25V threshold, the converter is disabled.
Error Amplifier
An internal high gain error amplifier is provided within the
LM5020. The amplifier’s non-inverting input is internally set
to a fixed reference voltage of 1.25V. The inverting input is
connected to the FB pin. In non-isolated applications, the
power converter output is connected to the FB pin via volt-
age scaling resistors. Loop compensation components are
connected between the COMP and FB pins. For most iso-
lated applications the error amplifier function is implemented
on the secondary side of the converter and the internal error
amplifier is not used. The internal error amplifier is config-
ured as an open drain output and can be disabled by con-
necting the FB pin to ground. An internal 5K pull-up resistor
between a 5V reference and COMP can be used as the
pull-up for an optocoupler in isolated applications.
Current Limit/Current Sense
The LM5020 provides a cycle-by-cycle over current protec-
tion function. Current limit is accomplished by an internal
current sense comparator. If the voltage at the current sense
comparator input exceeds 0.5V, the output is immediately
terminated. A small RC filter, located near the controller, is
recommended to filter noise from the current sense signal.
The CS input has an internal MOSFET which discharges the
CS pin capacitance at the conclusion of every cycle. The
discharge device remains on an additional 50ns after the
beginning of the new cycle to attenuate the leading edge
spike on the current sense signal.
The LM5020 current sense and PWM comparators are very
fast, and may respond to short duration noise pulses. Layout
considerations are critical for the current sense filter and
sense resistor. The capacitor associated with the CS filter
must be located very close to the LM5020 and connected
directly to the pins of the controller (CS and GND). If a
current sense transformer is used, both leads of the trans-
former secondary should be routed to the sense resistor and
the current sense filter network. A sense resistor located in
the source of the primary power MOSFET may be used for
current sensing, but a low inductance resistor is required.
When designing with a current sense resistor all of the noise
sensitive low power ground connections should be con-
nected together local to the controller and a single connec-
tion should be made to the high current power ground (sense
resistor ground point).