LM5035

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SNVS428H – JANUARY 2006 – REVISED OCTOBER 2015

Feature Description (continued)

7.3.4 Reference

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

external housekeeping circuits. The regulator output is internally current limited to 20-mA (typical).

7.3.5 Cycle-by-Cycle Current Limit

The CS pin is driven by a signal representative of the transformer primary current. If the voltage sensed at CS

pin exceeds 0.25 V, the current sense comparator terminates the HO or LO output driver pulse. If the high-

current condition persists, the controller operates in a cycle-by-cycle current-limit mode with duty cycle

determined by the current sense comparator instead of the PWM comparator. Cycle-by-cycle current limiting may

trigger the hiccup mode restart cycle, depending on the configuration of the RES pin (see the Overload

Protection Timer section).

TI recommends placing a small R-C filter connected to the CS pin and located near the controller to suppress

noise. An internal 32-

Ω MOSFET connected to the CS input discharges the external current sense filter capacitor

at the conclusion of every cycle. The discharge MOSFET remains on for an additional 50 ns after the HO or LO

driver switches high to blank leading edge transients in the current sensing circuit. Discharging the CS pin filter

each cycle and blanking leading edge spikes reduces the filtering requirements and improves the current sense

response time.

The current sense comparator is very fast and responds to short duration noise pulses. Layout considerations

are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be

placed very close to the device and must be connected directly to the CS and AGND pins. If a current sense

transformer is used, both leads of the transformer secondary must be routed to the filter network, which must be

located close to the IC. If a sense resistor located in the source of the main MOSFET switch is used for current

sensing, a low-inductance type of resistor is required. When designing with a current sense resistor, all of the

noise sensitive low-power ground connections must be connected together near the AGND pin, and a single

connection must be made to the power ground (sense resistor ground point).

7.3.6 Overload Protection Timer

The LM5035 provides a current-limit restart timer (see Figure 11) to disable the outputs and force a delayed

restart (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 programmable by the external capacitor at the RES pin. During each

PWM cycle, the LM5035 either sources or sinks current from the RES pin capacitor. If no current limit is detected

during a cycle, an 12-µA discharge current sink is enabled to pull the RES pin to ground. If a current limit is

detected, the 12-µA sink current is disabled and a 22-µA current source causes the voltage at the RES pin to

gradually increase. The LM5035 protects the converter with cycle-by-cycle current limiting while the voltage at

RES pin increases. If the RES voltage reaches the 2.5-V threshold, the following restart sequence occurs (also

see Figure 11):

1. The RES capacitor and SS capacitors are fully discharged.

2. The soft-start current source is reduced from 55 µA to 1 µA.

3. The SS capacitor voltage slowly increases. When the SS voltage reaches

≊1 V, the PWM comparator will

produce the first narrow output pulse. After the first pulse occurs, the SS source current reverts to the normal

55-µA level. The SS voltage increases at its normal rate, gradually increasing the duty cycle of the output

drivers.

4. If the overload condition persists after restart, cycle-by-cycle current limiting will begin to increase the voltage

on the RES capacitor again, repeating the hiccup mode sequence.

5. If the overload condition no longer exists after restart, the RES pin will be held at ground by the 12-µA

current sink and normal operation resumes.

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