LM5035B

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SNVS613C – JULY 2009 – REVISED APRIL 2013

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 LM5035B 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 LM5035B 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 LM5035B.

The operating input voltage range (VPWR) is 36V to 75V, and the 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 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 LM5035B VCC pin when the output is in regulation. The input

voltage UVLO thresholds are

≊34V for increasing VPWR, and ≊32V for decreasing VPWR. The circuit can be shut

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.

Table 2. Differences between LM5035, LM5035A, LM5035B, and LM5035C(1)(2)

Performance Feature:

LM5035

LM5035A

LM5035B

LM5035C

Sync Rectifier Dead-time Ratio (T1:T2)

2:1

3:1

3:1

3:1

Soft-start: Hiccup Mode Charging Current

50µA:1µA

100µA:1µA

100µA:1µA

100µA:1µA

Bootstrap (HB-HS) Under-Voltage Lockout

5V

3.9V

3.9V

3.9V

Start-up Regulator Current

20mA (min)

25mA (min)

40mA (min)

40mA (min)

SR State in UVLO Shutdown and Hiccup

High

High

Low

Low

Current Limit

HO,LO On-Time at Max Duty Cycle

0.5*T-T1–70 ns

0.5*T-T1–70 ns

0.5*T-T1

0.5*T-T1

Soft-Stop after UVLO

HO,LO

Yes

Yes

Yes

No

SR1,2

Yes

Yes

No

No

SR1, SR2 VOH (high state output)

VCC

VCC

VCC

REF (5V)

(1)

T1 = Delay from SR1, SR2 to leading edge of HO, LO

(2)

T

= Period of HO or LO

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