lm5035bsqx National Semiconductor Corporation, lm5035bsqx Datasheet - Page 16

• Manufacturer Part Number: lm5035bsqx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

www.national.com
Synchronous Rectifier Control
Outputs (SR1 & SR2)
Synchronous rectification (SR) of the transformer secondary
provides higher efficiency, especially for low output voltage
converters. The reduction of rectifier forward voltage drop
(0.5V - 1.5V) to 10mV - 200mV V
significantly reduces rectification losses. In a typical applica-
tion, the transformer secondary winding is center tapped, with
the output power inductor in series with the center tap. The
SR MOSFETs provide the ground path for the energized sec-
ondary winding and the inductor current. Figure 5 shows that
the SR2 MOSFET is conducting while HO enables power
transfer from the primary. The SR1 MOSFET must be dis-
abled during this period since the secondary winding con-
nected to the SR1 MOSFET drain is twice the voltage of the
center tap. At the conclusion of the HO pulse, the inductor
current continues to flow through the SR1 MOSFET body
diode. Since the body diode causes more loss than the SR
MOSFET, efficiency can be improved by minimizing the T2
period while maintaining sufficient timing margin over all con-
ditions (component tolerances, etc.) to prevent shoot-through
current. When LO enables power transfer from the primary,
the SR1 MOSFET is enabled and the SR2 MOSFET is off.
During the time that neither HO nor LO is active, the inductor
current is shared between both the SR1 and SR2 MOSFETs
which effectively shorts the transformer secondary and can-
cels the inductance in the windings. The SR2 MOSFET is
disabled before LO delivers power to the secondary to pre-
vent power being shunted to ground. The SR2 MOSFET body
diode continues to carry about half the inductor current until
the primary power raises the SR2 MOSFET drain voltage and
reverse biases the body diode. Ideally, dead-time T1 would
be set to the minimum time that allows the SR MOSFET to
turn off before the SR MOSFET body diode starts conducting.
DS
voltage for a MOSFET
FIGURE 5. HO, LO, SR1 and SR2 Timing Diagram
16
The SR1 and SR2 outputs are powered directly by the VCC
regulator. Each output is capable of sourcing and sinking 0.5A
peak. Typically, the SR1 and SR2 signals control SR MOS-
FET gate drivers through a pulse transformer. The actual gate
sourcing and sinking currents are provided by the secondary-
side bias supply and gate drivers.
The timing of SR1 and SR2 with respect to HO and LO is
shown in Figure 5. SR1 is configured out of phase with HO
and SR2 is configured out of phase with LO. The deadtime
between transitions is programmable by a resistor connected
from the DLY pin to the AGND pin. Typically, R
the range of 10kΩ to 100kΩ. The deadtime periods can be
calculated using the following formulae:
When UVLO falls below 1.25V, or during hiccup current limit,
both SR1 and SR2 are held low. During normal operation, if
soft-start is held low, both SR1 and SR2 will be high.
Thermal Protection
Internal Thermal Shutdown circuitry is provided to protect the
integrated circuit in the event the maximum rated junction
temperature is exceeded. When activated, typically at 165°C,
the controller is forced into a low power standby state with the
output drivers (HO, LO, SR1 and SR2), the bias regulators
(VCC and REF) disabled. This helps to prevent catastrophic
failures from accidental device overheating. During thermal
shutdown, the soft-start capacitor is fully discharged and the
controller follows a normal start-up sequence after the junc-
tion temperature falls to the operating level (145°C).
T2 = .0007 x R
T1 = .003 x R
DLY
DLY
+ 10.01 ns
+ 4.6 ns
30091321
DLY
is set in