lm5035bsqx National Semiconductor Corporation, lm5035bsqx Datasheet - Page 18

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

www.national.com
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 pe-
riod. The time average of this toggling is equivalent to a
continuous 5 µA current source into the RES capacitor, in-
creasing 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.
When using the resistor current sense method, an imbalance
in the input capacitor voltages may develop when operating
in cycle-by-cycle current limiting mode. If the imbalance per-
sists for an extended period, excessive currents in the non-
FIGURE 9. Current Sense Using Current Sense Resistor (R1)
FIGURE 8. Current Sense Using Current Sense Transformer
18
sensed MOSFET, and possible transformer saturation may
result. This condition is inherent to the half-bridge topology
operated with cycle-by-cycle current limiting and is com-
pounded by only sensing in one leg of the half-bridge circuit.
The imbalance is greatest at large duty cycles (low input volt-
ages). If using this method, it is recommended that the ca-
pacitor on the RES pin be no larger than 220 pF. Check the
final circuit and reduce the RES capacitor further, or omit the
capacitor completely to ensure the voltages across the bridge
capacitors remain balanced. The current limit value may de-
crease slightly as the RES capacitor is reduced.
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 surface
mount capacitor) should be connected close to the LM5035B,
between HB and HS to provide high peak currents during turn-
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