RES

SS

LO

HO

5V

t1

Current Limit Detected

at CS

2.5V

0V

+110 PA

+1 PA

t2

t3

#1V

LM5035C

www.ti.com

SNVS631C – JANUARY 2010 – REVISED MARCH 2013

Figure 28. Hiccup Over-Load Restart Timing

Printed Circuit Board Layout

The LM5035C Current Sense and PWM comparators are very fast, and respond to short duration noise pulses.

The components 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 resistor ground point).

The gate drive outputs of the LM5035C 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 LM5035C produces high junction 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 LM5035C 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 LM5035C.

capability is 30 Amps. Current sense transformer T2 provides information to the CS pin for current limit

protection. The error amplifier and reference, 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

winding on transformer T1 provides power to the LM5035C VCC pin when the output is in regulation. The input

voltage UVLO thresholds are

down by driving the ON/OFF input (J2) below 1.25V with an open-collector or open-drain circuit. An external

synchronizing frequency can be applied through a 100pF capacitor to the RT input (U1 pin 5). The regulator

output is current limited at

≊34A.

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