Datasheet

LM5026
SNVS363D –AUGUST 2005–REVISED APRIL 2013 www.ti.com
DETAILED OPERATING DESCRIPTION
The LM5026 PWM controller contains all of the features necessary to implement power converters utilizing the
active clamp reset technique with current mode control. With the active clamp reset, higher efficiencies and
greater power densities can be realized compared to conventional catch winding or RDC clamp reset techniques.
The LM5026 provides two control outputs, the main power switch control (OUT_A) and the active clamp switch
control (OUT_B). The device can be configured to drive either a P-Channel or N-Channel clamp switch. The
main switch gate driver features a compound configuration consisting of both MOS and bipolar devices, which
provide superior gate drive characteristics. The LM5026 can be configured to operate with bias voltages over a
wide input range from 8V to 100V. Additional features include programmable maximum duty cycle, line under-
voltage lockout, cycle-by-cycle current limit, hiccup mode fault protection with adjustable delays, PWM slope
compensation, soft-start, a 1MHz capable oscillator with synchronization Input / Output capability, precision
reference and thermal shutdown.
High Voltage Start-Up Regulator
The LM5026 contains an internal high voltage start-up regulator that allows the input pin (VIN) to be connected
directly to a nominal 48V dc line voltage. The regulator output (VCC) is internally current limited to 20mA. When
power is applied and the UVLO pin potential is greater than 0.4V, the regulator is enabled and sources current
into an external capacitor connected to the VCC pin. The recommended capacitance range for the VCC regulator
is 0.1µF to 100µF. The VCC regulator provides power to the internal voltage reference, PWM controller and gate
drivers. The controller outputs are enabled when the voltage on the VCC pin reaches the regulation point of
7.6V, the internal voltage reference (REF) reaches its regulation point of 5V and the UVLO voltage is greater
than 1.25V. In typical applications, an auxiliary transformer winding is connected through a diode to the VCC pin.
This winding must raise the VCC voltage above 8V to shut off the internal start-up regulator. Powering VCC from
an auxiliary winding improves efficiency while reducing the controller’s power dissipation.
The external VCC capacitor must be sized such that the current delivered from the capacitor and the VCC
regulator will maintain a VCC voltage greater than 6.2V during the initial start-up. During a fault mode when the
converter auxiliary winding is inactive, external current draw on the VCC line should be limited such that the
power dissipated in the start-up regulator does not exceed the maximum power dissipation of the IC package. An
external start-up or bias regulator can be used to power the LM5026 instead of the internal start-up regulator by
connecting the VCC and the VIN pins together and connecting an external bias supply to these two pins.
Line Under-Voltage Detector
The LM5026 contains a dual level Under-Voltage Lockout (UVLO) circuit. When the UVLO pin voltage is below
0.4V the controller is in a low current shutdown mode. When the UVLO pin voltage is greater than 0.4V but less
than 1.25V, the controller is in standby mode. In standby mode the VCC and REF bias regulators are active
while the controller outputs are disabled. When the VCC and REF outputs exceed the VCC and REF under-
voltage thresholds and the UVLO pin voltage is greater than 1.25V, the outputs are enabled and normal
operation begins. An external set-point voltage divider from VIN to GND can be used to set the operational range
of the converter. The divider must be designed such that the voltage at the UVLO pin will be greater than 1.25V
when VIN is in the desired operating range. UVLO hysteresis is accomplished with an internal 20uA current
source that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is
exceeded, 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 hysteresis of the 0.4V shutdown comparator is fixed at 100mV.
The UVLO pin can also be used to implement various remote enable / disable functions. Pulling the UVLO pin
below the 0.4V threshold totally disables the controller. Pulling the UVLO pin to a potential between 1.25 and
0.4V places the controller in standby with the VCC and REF regulators operating. Turning off a converter by
forcing the UVLO pin to the standby condition provides a controlled soft-stop. The controller outputs are not
directly disabled in standby mode, rather the soft-start capacitor is discharged with a 50µA sink current.
Discharging the soft-start capacitor gradually reduces the PWM duty cycle to zero, providing a slow controlled
discharge of the power converter output filter. This controlled discharge can help prevent uncontrolled behavior
of self-driven synchronous rectifiers during turn-off.
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