UCC38503

The UCC2850x family provides all of the control functions necessary for an active power-factor-corrected preregulator and a second-stage dc-to-dc converter. The controller achieves near-unity power factor by shaping the ac input line current waveform to correspond to the ac input-line voltage using average current-mode control. The dc-to-dc converter uses peak current-mode control to perform the step-down power conversion.

The PFC stage is leading-edge modulated while the second stage is trailing-edge synchronized to allow for minimum overlap between the boost and PWM switches. This reduces ripple current in the bulk-output capacitor.In order to operate with over three-to-one range of input-line voltages, a line feedforward (V_FF) is used to keep input power constant with varying input voltage. Generation of V_FF is accomplished using I_AC in conjunction with an external single-pole filter. This not only reduces external parts count, but also avoids the use of high-voltage components, offering a lower-cost solution. The multiplier then divides the line current by the square of V_FF.

The UCC2850x PFC section incorporates a low offset-voltage amplifier with 7.5-V reference, a highly-linear multiplier capable of a wide current range, a high-bandwidth, low offset-current amplifier, with a novel noise-attenuation configuration, PWM comparator and latch, and a high-current output driver. Additional PFC features include over-voltage protection, zero-power detection to turn off the output when VAOUT is below 0.33 V and peak current and power limiting.

The dc-to-dc section relies on an error signal generated on the secondary-side and processes it by performing peak current mode control. The dc-to-dc section also features current limiting, a controlled soft-start, preset operating range with selectable options, and 50% maximum duty cycle.

The UCC28500 and UCC28502 have a wide PFC-UVLO threshold (16.5 V/10 V) for bootstrap bias supply operation. The UCC28501 and UCC28503 are designed with a narrow UVLO range (10.5 V/10 V) more suitable for fixed bias operation. The UCC28500 and UCC28501 have a narrow UVLO threshold for PWM stage (to allow operation down to 75% of nominal bulk voltage), while the UCC28502 and UCC38503 are configured for a much wider operation range for the PWM stage (down to 50% of bulk nominal voltage).

Available in 20-pin N and DW packages.