LM 5027A

VIN

VCC

:

50

F

P

1

.

0

power stage

9V-15V (from

)

OUTA

OUTB

SR

Edge

PWM

T1

Edge

Delay_ Leading

T2

T3

Delay_ trailing

LM5027A

www.ti.com

SNVS642B – APRIL 2010 – REVISED MARCH 2013

Figure 26. LM5027A Driver Output Timing

Thermal Protection

Internal Thermal Shutdown circuitry is provided to protect the integrated circuit in the event the maximum rated

junction temperature is exceeded. When activated, typically at 165°C, the controller is forced into a low power

standby state with the output drivers (OUTA, OUTB, and OUTSR), the bias regulators (VCC and REF) disabled.

The thermal protection feature is provided to prevent catastrophic failures from accidental device overheating.

During a restart, after thermal shutdown, the soft-start capacitors (SS and SSSR) are fully discharged and the

controller follows a normal start-up sequence after the junction temperature falls below the Thermal Shutdown

hysteresis threshold (typically 145°C).

VIN

The voltage applied to the VIN pin, normally the same as the system voltage applied to the power transformer’s

depends primarily on the output driver capacitive loads, the switching frequency, and any external loads on the

VCC pin. If the power dissipation associated with the VIN current exceeds the package capability, an external

voltage should be applied to the VCC pin (see Figure 27) to disable the internal start-up regulator. The VCC

regulation set point voltage is initially internally regulated to 9.5V. After the first OUTSR pulse, the VCC set point

voltage is reduced to 7.5V. If an external voltage is applied to the VCC pin the required range is 8V to 15V . The

VIN to VCC series pass regulator includes a parasitic diode between VIN and VCC. This diode should not be

forward biased in normal operation. The VCC voltage should never exceed the VIN voltage. It is recommended

the circuit of Figure 27 be used to suppress transients which may occur at the input supply, in particular where

VIN is operated close to the maximum operating rating of the LM5027A.

Figure 27. Start-up Regulator Power Reduction

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