lm5035mhx National Semiconductor Corporation, lm5035mhx Datasheet - Page 18

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

www.national.com
Applications Information
If the current sense resistor method is used, the over-current
condition will only be sensed while LO is driving the low-side
MOSFET. Over-current while HO is driving the high-side
MOSFET will not be detected. In this configuration, it will
take 4 times as long for continuous cycle-by-cycle current
limiting to initiate a restart event since each over-current
event during LO enables the 22µA RES pin current source
for one oscillator period, and then the lack of an over-current
event during HO enables the 12µA RES pin current sink for
one oscillator period. The time average of this toggling is
equivalent to a continuous 5µA current source into the RES
capacitor, increasing the delay by a factor of four. The value
of the RES capacitor can be reduced to decrease the time
before restart cycle is initiated.
HO, HB, HS and LO
Attention must be given to the PC board layout for the
low-side driver and the floating high-side driver pins HO, HB
and HS. A low ESR/ESL capacitor (such as a ceramic sur-
face mount capacitor) should be connected close to the
LM5035, between HB and HS to provide high peak currents
during turn-on of the high-side MOSFET. The capacitor
should be large enough to supply the MOSFET gate charge
FIGURE 9. Current Sense Using Current Sense Resistor (R1)
FIGURE 8. Current Sense Using Current Sense Transformer
(Continued)
18
(Qg) without discharging to the point where the drop in gate
voltage affects the MOSFET R
times Qg is recommended.
The diode (D
the low-side MOSFET is conducting should be capable of
withstanding the full converter input voltage range. When the
high-side MOSFET is conducting, the reverse voltage at the
diode is approximately the same as the MOSFET drain
voltage because the high-side driver is boosted up to the
converter input voltage by the HS pin, and the high side
MOSFET gate is driven to the HS voltage plus VCC. Since
the anode of D
potential across the diode is equal to the input voltage minus
the VCC voltage. D
in most applications, so a low current ultra-fast recovery
diode is recommended to limit the loss due to diode junction
BOOST
BOOST
BOOST
) that charges C
20177525
20177524
is connected to VCC, the reverse
average current is less than 20mA
DS(ON)
BOOST
. A value ten to twenty
from VCC when