LM5045

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SNVS699G – FEBRUARY 2011 – REVISED MARCH 2013

Fault Condition

SSSR

UVLO

Soft-Stop

(UVLO<1.25V)

3x the charging rate

OVP

Hard-Stop

(OVP>1.25V)

Hiccup

Soft-Stop

(CS>0.75 and RES>1V)

6x the charging rate

VCC/VREF UV

Hard-Stop

Internal Thermal Limit

Hard-Stop

(1)

Note: All the above conditions are valid with SSOFF pin tied to GND. If SSOFF=5V, the LM5045 hard-stops in all the conditions. The SS

pin remains high in all the conditions until the SSSR pin reaches 1V.

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 160°C, the controller is forced into a shutdown

state with the output drivers, the bias regulators (VCC and REF) disabled. This helps to prevent catastrophic

failures from accidental device overheating. During thermal shutdown, the SS and SSSR capacitors are fully

discharged and the controller follows a normal start-up sequence after the junction temperature falls to the

operating level (140 °C).

APPLICATIONS INFORMATION

Control Method Selection

The LM5045 is a versatile PWM control IC that can be configured for either current mode control or voltage

mode control. The choice of the control method usually depends upon the designer preference. The following

must be taken into consideration while selecting the control method. Current mode control can inherently balance

flux in both phases of the full-bridge topology. The full-bridge topology, like other double ended topologies, is

susceptible to the transformer core saturation. Any asymmetry in the volt-second product applied between the

two alternating phases results in flux imbalance that causes a dc buildup in the transformer. This continual dc

buildup may eventually push the transformer into saturation. The volt-second asymmetry can be corrected by

employing current mode control. In current mode control, a signal representative of the primary current is

compared against an error signal to control the duty cycle. In steady-state, this results in each phase being

terminated at the same peak current by adjusting the pulse-width and thus applying equal volt-seconds to both

the phases.

Current mode control can be susceptible to noise and sub-harmonic oscillation, while voltage mode control

employs a larger ramp for PWM and is usually less susceptible. Voltage-mode control with input line feed-

forward also has excellent line transient response. When configuring for voltage mode control, a dc blocking

capacitor can be placed in series with the primary winding of the power transformer to avoid any flux imbalance

that may cause transformer core saturation.

Voltage Mode Control Using the LM5045

VIN, AGND, and the RAMP pins is required to create a saw-tooth modulation ramp signal shown in Figure 19.

The slope of the signal at RAMP will vary in proportion to the input line voltage. The varying slope provides line

feed-forward information necessary to improve line transient response with voltage mode control. With a constant

of the transformer primary. Using a line feed-forward ramp for PWM control requires very little change in the

voltage regulation loop to compensate for changes in input voltage, as compared to a fixed slope oscillator ramp.

Furthermore, voltage mode control is less susceptible to noise and does not require leading edge filtering.

Therefore, it is a good choice for wide input range power converters. Voltage mode control requires a Type-III

compensation network, due to the complex-conjugate poles of the L-C output filter.

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Product Folder Links: LM5045