UCC28C53_V01 datasheet(29/48 Pages) TI

29 / 48 page

9.2.2.5 Current Sensing Network

The current sensing network consists of the primary-side current sensing resistor (RCS), filtering components

RCSF and CCSF, and optional RP. Typically, the direct current sense signal contains a large amplitude leading

edge spike associated with the turnon of the main power MOSFET, reverse recovery of the output rectifier, and

other factors including charging and discharging of parasitic capacitances. Therefore, CCSF and RCSF form a

low-pass filter that provides immunity to suppress the leading edge spike. For this converter, CCSF is chosen to

be 100 pF.

Without RP, RCS sets the maximum peak current in the transformer primary based on the maximum amplitude of

the CS pin, which is specified to be 1 V. To achieve 1.36-A primary side peak current, a 0.75-Ω resistor is chosen

for RCS.

The high current sense threshold of CS helps to provide better noise immunity to the system but also results

in higher losses in the current sense resistor. These current sense losses can be minimized by injecting an

offset voltage into the current sense signal using RP. RP and RCSF form a resistor divider network from the

current sense signal to the reference voltage of the controller (VVREF) which adds an offset to the current

sense voltage. This technique still achieves current mode control with cycle-by-cycle over-current protection. To

calculate required offset value (VOFFSET), use Equation 16.

OFFSET

CSF

CSF + RP

× V

REF

(16)

After adding the RP resistance, adjust the RCS value accordingly.

9.2.2.6 Gate Drive Resistor

RG is the gate driver resistor for the power switch (QSW). The selection of this resistor value must be done in

conjunction with EMI compliance testing and efficiency testing. Using a larger resistor value for RG slows down

the turnon and turnoff of the MOSFET. A slower switching speed reduces EMI but also increases the switching

loss. A tradeoff between switching loss and EMI performance must be carefully performed. For this design, a

‑Ω resistor was chosen for the gate drive resistor.

9.2.2.7 VREF Capacitor

A precision 5-V reference voltage performs several important functions. The reference voltage is divided down

internally to 2.5 V and connected to the error amplifier’s noninverting input for accurate output voltage regulation.

Other duties of the reference voltage are to set internal bias currents and thresholds for functions such as

the oscillator upper and lower thresholds. Therefore, the reference voltage must be bypassed with a ceramic

capacitor. A 1-µF, 16-V ceramic capacitor was selected for this converter. Placement of this capacitor on the

physical printed-circuit board layout must be as close as possible to the respective VREF and GND pins.

9.2.2.8 RT/CT

RT/CT is the oscillator timing pin. For fixed frequency operation, set the timing capacitor charging current by

connecting a resistor from VREF to RT/CT. Set the frequency by connecting timing capacitor from RT/CT to

GND. For the best performance, keep the timing capacitor lead to GND as short and direct as possible. If

possible, use separate ground traces for the timing capacitor and all other functions.

The controller's oscillator allows for operation to 1 MHz. The device uses an external resistor to set the charging

current for the external capacitor, which determines the oscillator frequency. TI recommends timing resistor

values from 1 kΩ to 100 kΩ and timing capacitor values from 220 pF to 4.7 nF. The UCCx8C5x oscillator is true

to the curves of the original BiCMOS devices at lower frequencies, yet extends the frequency programmability

range to at least 1 MHz. This programmability allows the device to offer pin-to-pin capability where required, yet

capable of extending the operational range to the higher frequencies.

See Figure 7-1 for component values for setting the oscillator frequency.

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SLUSER8 – JUNE 2022

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