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LM5045

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SNVS699G – FEBRUARY 2011 – REVISED MARCH 2013

Over Voltage Protection

An external voltage divider can be used to set either an over voltage or an over temperature protection. During

an OVP condition, the SS and SSSR capacitors are discharged and all the outputs are disabled. The divider

must be designed such that the voltage at the OVP pin is greater than 1.25V when over voltage/temperature

condition exists. Hysteresis is accomplished with an internal 20

μA current source. When the OVP pin voltage

exceeds 1.25V, the 20

μA current source is activated to quickly raise the voltage at the OVP pin. When the OVP

pin voltage falls below the 1.25V threshold, the current source is deactivated causing the voltage at the OVP to

quickly fall. Refer to the APPLICATIONS INFORMATION section for more details.

Reference

The REF pin is the output of a 5V linear regulator that can be used to bias an opto-coupler transistor and

external housekeeping circuits. The regulator output is internally current limited to 15mA. The REF pin needs to

be locally decoupled with a ceramic capacitor, the recommended range of values are from 0.1

μF to 10μF

Oscillator, Sync Input

The LM5045 oscillator frequency is set by a resistor connected between the RT pin and AGND. The RT resistor

RT resistor can be calculated from the following equation:

(1)

For example, if the desired oscillator frequency is 400 kHz i.e. each phase (LO1 or LO2) at 200 kHz, the value of

RT pin through a 100pF capacitor. The RT pin voltage is nominally regulated at 2.0V and the external pulse

amplitude should lift the pin to between 3.5V and 5.0V on the low-to-high transition. The synchronization pulse

width should be between 15 and 200ns. The RT resistor is always required, whether the oscillator is free running

or externally synchronized and the SYNC frequency must be equal to, or greater than the frequency set by the

RT resistor. When syncing to an external clock, it is recommended to add slope compensation by connecting an

appropriate resistor from the VCC pin to the CS pin. Also disable the SLOPE pin by grounding it.

Cycle-by-Cycle Current Limit

The CS pin is to be driven by a signal representative of the transformer’s primary current. If the voltage on the

CS pin exceeds 0.75V, the current sense comparator immediately terminates the PWM cycle. A small RC filter

connected to the CS pin and located near the controller is recommended to suppress noise. An internal 18

Ω

MOSFET discharges the external current sense filter capacitor at the conclusion of every cycle. The discharge

MOSFET remains on for an additional 40ns after the start of a new PWM cycle to blank leading edge spikes. The

current sense comparator is very fast and may respond to short duration noise pulses. Layout is critical for the

current sense filter and the sense resistor. The capacitor associated with CS filter must be placed very close to

the device and connected directly to the CS and AGND pins. If a current sense transformer is used, both the

leads of the transformer secondary should be routed to the filter network, which should be located close to the

IC. When designing with a current sense resistor, all of the noise sensitive low power ground connections should

be connected together near the AGND pin, and a single connection should be made to the power ground (sense

resistor ground point).

Hiccup Mode

The LM5045 provides a current limit restart timer to disable the controller outputs and force a delayed restart (i.e.

Hiccup mode) if a current limit condition is repeatedly sensed. The number of cycle-by-cycle current limit events

required to trigger the restart is programmed by the external capacitor at the RES pin. During each PWM cycle,

the LM5045 either sources or sinks current from the RES capacitor. If current limit is detected, the 5

μA current

sink is disabled and a 30

μA current source is enabled. If the RES voltage reaches the 1.0V threshold, the

following restart sequence occurs, as shown in Figure 12:

•

The SS and SSSR capacitors are fully discharged

•

The 30

μA current source is turned-off and the 10μA current source is turned-on.

•

Once the voltage at the RES pin reaches 4.0V the 10

μA current source is turned-off and a 5μA current sink is

turned-on, ramping the voltage on the RES capacitor down to 2.0V.

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