UCC28C53_V01 datasheet(17/48 Pages) TI

17 / 48 page

Table 8-2. UVLO Options

MAXIMUM

DUTY CYCLE (%)

UVLO ON

(V)

UVLO OFF

(V)

DEVICE

NUMBER

100

14.5

UCCx8C52

100

8.4

7.6

UCCx8C53

100

6.6

UCCx8C50

100

18.8

15.5

UCC28C56H

100

18.8

14.5

UCC28C56L

100

12.5

UCC28C58

14.5

UCCx8C54

8.4

7.6

UCCx8C55

6.6

UCCx8C51

18.8

15.5

UCC28C57H

18.8

14.5

UCC28C57L

12.5

UCC28C59

During UVLO the IC draws less than 75 µA of supply current. Once crossing the turnon threshold the IC supply

current increases to a maximum of 2mA, typically 1.3 mA. This low start-up current allows the power supply

designer to optimize the selection of the startup resistor value to provide a more efficient design. In applications

where low component cost overrides maximum efficiency, the low run current of 1.3 mA (typical) allows the

control device to run directly through the single resistor to (+) rail, rather than requiring a bootstrap winding on

the power transformer, along with a rectifier. The start and run resistor for this case must also pass enough

current to allow driving the primary switching MOSFET, which may be a few milliamps in small devices.

< 0.075

VOFF

< 2

VON

Operating Voltage (V)

Figure 8-2. UVLO ON and OFF Profile

8.3.3 ±1% Internal Reference Voltage

The BiCMOS internal reference of 2.5 V has an enhanced design, and uses production trim to allow initial

accuracy of ±1% at room temperature and ±2% over the full temperature range. This reference voltage can be

used to eliminate an external reference in applications that do not require the extreme accuracy afforded by the

additional device. This reference voltage is useful for non-isolated DC-DC applications, where the control device

is referenced to the same common as the output. It is also applicable in off-line designs that regulate on the

primary side of the isolation boundary by looking at a primary bias winding, or from a winding on the output

inductor of a buck-derived circuit.

8.3.4 Current Sense and Overcurrent Limit

An external series resistor (RCS) senses the current and converts this current into a voltage that becomes the

input to the CS pin. The CS pin is the noninverting input to the PWM comparator. The device compares the CS

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UCC28C53, UCC28C54, UCC28C55

UCC28C56H, UCC28C57H, UCC28C57L, UCC28C59

SLUSER8 – JUNE 2022

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