lm5035mhx National Semiconductor Corporation, lm5035mhx Datasheet - Page 11

• Manufacturer Part Number: lm5035mhx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

Functional Description
The LM5035 PWM controller contains all of the features
necessary to implement half-bridge voltage-mode controlled
power converters. The LM5035 provides two gate driver
outputs to directly drive the primary side power MOSFETs
and two signal level outputs to control secondary synchro-
nous rectifiers through an isolation interface. Secondary side
drivers, such as the LM5110, are typically used to provide
the necessary gate drive current to control the sync MOS-
FETs. Synchronous rectification allows higher conversion
efficiency and greater power density than conventional PN or
Schottky rectifier techniques. The LM5035 can be configured
to operate with bias voltages ranging from 8V to 105V.
Additional features include line under-voltage lockout, cycle-
by-cycle current limit, voltage feed-forward compensation,
hiccup mode fault protection with adjustable delays, soft-
start, a 2MHz capable oscillator with synchronization capa-
bility, precision reference, thermal shutdown and program-
mable volt • second clamping. These features simplify the
design of voltage-mode half-bridge DC-DC power convert-
ers. The Functional Block Diagram is shown in Figure 1.
High-Voltage Start-up Regulator
The LM5035 contains an internal high voltage start-up regu-
lator that allows the input pin (VIN) to be connected directly
to a nominal 48 VDC input voltage. The regulator input can
withstand transients up to 105V. The regulator output at VCC
(7.6V) is internally current limited to 25 mA typical. When the
UVLO pin potential is greater than 0.4V, the VCC regulator is
enabled to charge an external capacitor connected to the
VCC pin. The VCC regulator provides power to the voltage
reference (REF) and the output drivers (LO, SR1 and SR2).
When the voltage on the VCC pin exceeds the UVLO thresh-
old of 7.6V, the internal voltage reference (REF) reaches its
regulation setpoint of 5V and the UVLO voltage is greater
than 1.25V, the controller outputs are enabled. The value of
the VCC capacitor depends on the total system design, and
its start-up characteristics. The recommended range of val-
ues for the VCC capacitor is 0.1µF to 100µF.
The VCC under-voltage comparator threshold is lowered to
6.2V (typical) after VCC reaches the regulation set-point. If
VCC falls below this value, the outputs are disabled, and the
soft-start capacitor is discharged. If VCC increases above
7.6V, the outputs will be enabled and a soft-start sequence
will commence.
The internal power dissipation of the LM5035 can be re-
duced by powering VCC from an external supply. In typical
applications, an auxiliary transformer winding is connected
through a diode to the VCC pin. This winding must raise the
VCC voltage above 8.3V to shut off the internal start-up
regulator. Powering VCC from an auxiliary winding improves
efficiency while reducing the controller’s power dissipation.
The under-voltage comparator circuit will still function in this
mode, requiring that VCC never falls below 6.2V during the
start-up sequence.
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 regula-
tor does not exceed the maximum power dissipation of the
IC package.
An external DC bias voltage can be used instead of the
internal regulator by connecting the external bias voltage to
11
both the VCC and the VIN pins. The external bias must be
greater than 8.3V to exceed the VCC UVLO threshold and
less than the VCC maximum operating voltage rating (15V).
Line Under-Voltage Detector
The LM5035 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 out-
puts 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 minimum
operating voltage of the converter. The divider must be
designed such that the voltage at the UVLO pin will be
greater than 1.25V when VIN enters the desired operating
range. UVLO hysteresis is accomplished with an internal
23uA current sink that is switched on or off into the imped-
ance of the set-point divider. When the UVLO threshold is
exceeded, the current sink is deactivated to quickly raise the
voltage at the UVLO pin. When the UVLO pin voltage falls
below the 1.25V threshold, the current sink is enabled caus-
ing the voltage at the UVLO pin to quickly fall. The hysteresis
of the 0.4V shutdown comparator is internally fixed at
100mV.
The UVLO pin can also be used to implement various re-
mote enable / disable functions. Turning off a converter by
forcing the UVLO pin to the standby condition provides a
controlled soft-stop. See the Soft Start section for more
details.
Line Over Voltage / Load Over
Voltage / Remote Thermal
Protection
The LM5035 provides a multipurpose OVP pin that supports
several fault protection functions. When the OVP pin voltage
exceeds 1.25V, the controller is held in standby mode which
immediately halts the PWM pulses at the HO and LO pins. In
standby mode, the VCC and REF bias regulators are active
while the controller outputs are disabled. When the OVP pin
voltage falls below the 1.25V OVP threshold, the outputs are
enabled and normal soft-start sequence begins. Hysteresis
is accomplished with an internal 23µA current source that is
switched on or off into the impedance of the OVP pin set-
point divider. When the OVP threshold is exceeded, the
current source is enabled to quickly raise the voltage at the
OVP pin. When the OVP pin voltage falls below the 1.25V
threshold, the current source is disabled causing the voltage
at the OVP pin to quickly fall.
Several examples of the use of this pin are provided in the
Application Information section.
Reference
The REF pin is the output of a 5V linear regulator that can be
used to bias an opto-coupler transistor and external house-
keeping circuits. The regulator output is internally current
limited to 20mA (typical).
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