Datasheet

CLK

OUTA

OUTB

ILIM

70ns

0.5V

UCC28250

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SLUSA29C –APRIL 2010– REVISED JULY 2011

Cycle-by-Cycle Current Limit and Hiccup Mode Protection

Cycle-by-cycle current limit is accomplished using the ILIM pin for both current mode control and voltage mode

control. The input to the ILIM pin represents the primary current information. If the voltage sensed at ILIM pin

exceeds 0.5 V, the current sense comparator terminates the pulse of output OUTA or OUTB. 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. ILIM pin is pulled down by an internal switch at the

rising edge of each clock cycle. This internal switch remains on for an additional 70 ns after OUTA or OUTB

goes high to blank leading edge transient noise in the current sensing loop. This reduces the filtering

requirements at the ILIM pin and improves the current sense response time.

UCC28250 makes it possible to maintain flux balance during cycle-by-cycle current limit operation. The duty

cycles of primary switches are always matched. If one switch duty cycle is terminated earlier because of current

limiting, a matched duty cycle is applied to the other switch for the next half switching cycle, regardless of the

current condition, as shown in Figure 44. This matched duty cycle helps to maintain volt-second balancing on the

transformer and prevents the transformer saturation.

Figure 44. Cycle-by-Cycle Current Limit Duty Cycle Matching

Once the current limit is triggered, the 75-µA internal current source begins to charge the capacitor on HICC pin.

If the current limit condition went away before HICC pin reaches 0.6 V, the device stops charge HICC capacitor

and begins to discharge it with 2.7-µA current source. If the cycle-by-cycle current limit condition continues, HICC

pin reachs 0.6 V, and all four outputs are shut down. The UCC28250 then enters hiccup mode. During hiccup

mode, all four outputs keep low; SS pin is pulled to ground internally; a 2.7-µA current source continuously

discharge HICC pin capacitor; until HICC pin voltage reaches 0.3 V. After that, HICC pin is discharged internally

to get ready for the next HICC event. The whole converter starts with soft start after hiccup mode.

Product Folder Link(s): UCC28250