lm5035mhx National Semiconductor Corporation, lm5035mhx Datasheet - Page 23

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

Applications Information
In the case of continuous cycle-by-cycle current limit detec-
tion at the CS pin, the time required for C
2.5V hiccup mode threshold is:
For example, if C
1.14 ms.
The cool down time, t2 is set by the soft-start capacitor (C
and the internal 1µA SS current source, and is equal to:
If C
The soft-start time t3 is set by the internal 55µA current
source, and is equal to:
If C
The time t2 provides a periodic cool-down time for the power
converter in the event of a sustained overload or short
circuit. This off time results in lower average input current
and lower power dissipation within the power components. It
is recommended that the ratio of t2 / (t1 + t3) be in the range
of 5 to 10 to take advantage of this feature.
If the application requires no delay from the first detection of
a current limit condition to the onset of the hiccup mode (t1 =
0), the RES pin can be left open (no external capacitor). If it
is desired to disable the hiccup mode entirely, the RES pin
should be connected to ground (AGND).
Printed Circuit Board Layout
The LM5035 Current Sense and PWM comparators are very
fast, and respond to short duration noise pulses. The com-
SS
SS
= 0.01 µF t2 is )10 ms.
= 0.01 µF t3 is )730µs.
RES
= 0.01µF the time t1 is approximately
FIGURE 16. Hiccup Over-Load Restart Timing
RES
(Continued)
to reach the
SS
)
23
ponents at the CS, COMP, SS, OVP, UVLO, DLY and the RT
pins should be as physically close as possible to the IC,
thereby minimizing noise pickup on the PC board tracks.
Layout considerations are critical for the current sense filter.
If a current sense transformer is used, both leads of the
transformer secondary should be routed to the sense filter
components and to the IC pins. The ground side of the
transformer should be connected via a dedicated PC board
track to the AGND pin, rather than through the ground plane.
If the current sense circuit employs a sense resistor in the
drive transistor source, low inductance resistors should be
used. In this case, all the noise sensitive, low-current ground
tracks should be connected in common near the IC, and then
a single connection made to the power ground (sense resis-
tor ground point).
The gate drive outputs of the LM5035 should have short,
direct paths to the power MOSFETs in order to minimize
inductance in the PC board traces. The SR control outputs
should also have minimum routing distance through the
pulse transformers and through the secondary gate drivers
to the sync FETs.
The two ground pins (AGND, PGND) must be connected
together with a short, direct connection, to avoid jitter due to
relative ground bounce.
If the internal dissipation of the LM5035 produces high junc-
tion temperatures during normal operation, the use of mul-
tiple vias under the IC to a ground plane can help conduct
heat away from the IC. Judicious positioning of the PC board
within the end product, along with use of any available air
flow (forced or natural convection) will help reduce the junc-
tion temperatures. If using forced air cooling, avoid placing
the LM5035 in the airflow shadow of tall components, such
as input capacitors.
Application Circuit Example
The following schematic shows an example of a 100W half-
bridge power converter controlled by the LM5035. The op-
erating input voltage range (V
output voltage is 3.3V. The output current capability is 30
Amps. Current sense transformer T2 provides information to
the CS pin for current limit protection. The error amplifier and
PWR
) is 36V to 75V, and the
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