UCC28C53_V01 datasheet(15/48 Pages) TI

15 / 48 page

8.3.1 Detailed Pin Description

8.3.1.1 COMP

The error amplifier in the UCCx8C5x family has a unity-gain bandwidth of 1 MHz. The COMP terminal can both

source and sink current. The error amplifier is internally current-limited, so that one can command zero duty

cycle by externally forcing COMP to GND.

8.3.1.2 FB

FB is the inverting input of the error amplifier. The noninverting input to the error amplifier is internally trimmed to

2.5 V ± 1%. FB is used to control the power converter voltage-feedback loop for stability. For best stability, keep

FB lead length as short as possible and FB stray capacitance as small as possible.

8.3.1.3 CS

The UCCx8C5x current sense input connects directly to the PWM comparator. Connect CS to the MOSFET

source current sense resistor. The PWM uses this signal to terminate the OUT switch conduction. A voltage

ramp can be applied to this pin to run the device with a voltage mode control configuration or to add slope

compensation. To prevent false triggering due to leading edge noises, an RC current sense filter may be

required. The gain of the current sense amplifier is typically 3 V/V.

8.3.1.4 RT/CT

The internal oscillator uses a timing capacitor (CCT) and a timing resistor (RRT) to program the oscillator

frequency and maximum duty cycle. The operating frequency can be programmed based the curves in Figure

7-1, where the timing resistor can be found once the timing capacitor is selected. It is best for the timing

capacitor to have a flat temperature coefficient, typical of most COG or NPO type capacitors. For this converter,

15.4 kΩ and 1000 pF were selected for RRT and CCT to operate at 110-kHz switching.

8.3.1.5 GND

GND is the signal and power returning ground. TI recommends separating the signal return path and the high

current gate driver path so that the signal is not affected by the switching current.

8.3.1.6 OUT

The high-current output stage of the UCCx8C5x to drive the external power switch has been kept the same as

the earlier devices UCC28C4x. To drive a power MOSFET directly, the totem-pole OUT driver sinks or source

up to 1 A peak of current. The OUT of the UCCx8C50, UCCx8C52, UCCx8C53 UCC28C56H/L and UCC28C58

devices switch at the same frequency as the oscillator and can operate near 100% duty cycle. In the UCCx8C51,

UCCx8C54, and UCCx8C55, UCC28C57H/L and UCC28C59, the switching frequency of OUT is one-half that

of the oscillator due to an internal T flip-flop. This limits the maximum duty cycle in the UCCx8C51, UCCx8C54,

and UCCx8C55, UCC28C57H/L and UCC28C59 to < 50%.

The UCCx8C5x family houses unique totem pole drivers exhibiting a 10-Ω impedance to the upper rail and a

5.5

‑Ω impedance to ground, typically. This reduced impedance on the low-side switch helps minimize turn-off

losses at the power MOSFET, whereas the higher turnon impedance of the high-side is intended to better match

the reverse recovery characteristics of many high-speed output rectifiers. Transition times, rising and falling

edges, are typically 25 nanoseconds and 20 nanoseconds, respectively, for a 10% to 90% change in voltage.

A low impedance MOS structure in parallel with a bipolar transistor, or BiCMOS construction, comprises the

totem-pole output structure. This more efficient utilization of silicon delivers the high peak current required along

with sharp transitions and full rail-to-rail voltage swings. Furthermore, the output stage is self-biasing, active

low during under-voltage lockout type. With no VDD supply voltage present, the output actively pulls low if

an attempt is made to pull the output high. This condition frequently occurs at initial power-up with a power

MOSFET as the driver load.

8.3.1.7 VDD

VDD is the power input connection for this device. In normal operation, power VDD through a current limiting

resistor. The absolute maximum supply voltage is 30 V (extended from 20 V of UCCx8C4x) to facilitate designs

and applications. The voltage level of 30 V, including any transients that may be present, cannot be exceeded,

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UCC28C53, UCC28C54, UCC28C55

UCC28C56H, UCC28C57H, UCC28C57L, UCC28C59

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