Datasheet

LM5045
www.ti.com
SNVS699G FEBRUARY 2011REVISED MARCH 2013
Table 1. Soft-Stop in Fault Conditions
(1)
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
To configure the LM5045 for voltage mode control, an external resistor (R
FF
) and capacitor (C
FF
) connected to
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
error signal, the on-time (T
ON
) varies inversely with the input voltage (VIN) to stabilize the Volt- Second product
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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