Low-Side Buck Stage

(

)

SW min

PK

PK

IN

OUT

OUT

1

f

L I

L I

V

V

V

@

æ

ö

æ

ö

´

´

+

ç

÷

ç

÷

-

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ø

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(4)

UCC28810

UCC28811

The LED current is provided by a low-side buck converter operating in critical conduction mode, shown in

Figure 29. The buck controller is programmed to provide a peak current of two times (2 ×) the maximum average

load current of 0.9 A (nom). The critical conduction mode allows the current to reach 0 A before beginning the

next cycle. This method provides high-efficiency due to minimized voltage on the MOSFET drain at turn on.

Cycle-by-cycle current control to the LEDs is also a benefit of critical conduction mode. The average current of

the buck driver is controlled via the PWM input (J9, Pin 3).

The UCC28811 is configured to operate in peak current limit mode with the ability to shutdown and PWM control

the buck converter with the enable function on the VSENSE pin.

The voltage divider formed by R27 and R28 from the 5.1-V zener diode (D9) provides approx 2 V to VSENSE

which is below the internal reference and above the enable threshold. R29 and R30 is a divider which biases

VINS at approx 3 V.

The saturated EAOUT and VINS saturates the current reference generator so the UCC28811 VDRV termination

times the desired average LED current

The minimum operating frequency of a given inductor value can be determined by summing the on and off time

of the buck switch to achieve the desired peak-to-peak current.

R40 and R48 provide a small current from LED+ into the ISENSE filter to offset the change in peak current due

determined. The delta in di/dt results in a

Overvoltage protection is provided to protect against open circuit loads, shown in Figure 27. The circuit provides

detection of voltage between LED+ and LED– without a current path from LED- to ground in normal operation.

The trigger voltage is determined by the total zener voltage of D15 and D19, (150 V) in this example. Once the

Q6 is divided down with R44 and R42 and summed into the buck shutdown through D14.

A undervoltage lockout circuit is recommended for low-side buck LED current sources operating at output

voltages over 115 V. A simple, effective UVLO circuit is shown in Figure 28. When the 2N2222 transistor is off,

the collector is pulled high which disables the buck convertor through the common shutdown path . When the

total zener voltage is exceeded, the 2N2222 is turned on enabling the buck converter. The UVLO enable voltage

should be selected to be between the highest anticipated buck output and the minimum output voltage of the

PFC boost follower.

Copyright © 2008, Texas Instruments Incorporated

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