LM5035EVAL National Semiconductor, LM5035EVAL Datasheet - Page 23

• Manufacturer Part Number: LM5035EVAL
• Description: BOARD EVALUATION LM5035
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5035EVAL

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Isolated
• Voltage - Output: 3.3V
• Current - Output: 30A
• Voltage - Input: 36 ~ 75V
• Regulator Topology: Buck
• Frequency - Switching: 400kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5035
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -

In the case of continuous cycle-by-cycle current limit detec-
tion at the CS pin, the time required for C
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 recom-
mended 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 compo-
nents at the CS, COMP, SS, OVP, UVLO, DLY and the RT
SS
SS
= 0.01 µF t2 is
= 0.01 µF t3 is
RES
≊
≊
= 0.01µF the time t1 is approximately
10 ms.
730µs.
FIGURE 16. Hiccup Over-Load Restart Timing
RES
to reach the 2.5V
SS
)
23
pins should be as physically close as possible to the IC, there-
by 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 trans-
former secondary should be routed to the sense filter com-
ponents 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 resistor
ground point).
The gate drive outputs of the LM5035 should have short, di-
rect paths to the power MOSFETs in order to minimize in-
ductance 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 to-
gether 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 multiple
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 junction
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 oper-
ating input voltage range (V
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 refer-
ence, U3 and U5 respectively, provide voltage feedback via
opto-coupler U4. Synchronous rectifiers Q4, Q5, Q6 and Q7
minimize rectification losses in the secondary. An auxiliary
PWR
) is 36V to 75V, and the output
20177516
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