TLC5960DA datasheet(21/33 Pages) TI

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LED

R1+R2

´ V

REF

´ (V

)

REF -

HVM

LEDMIN

R1+R2

´ V

REF

´ (V

1.25)

REF -

LEDTYP

R1+R2

´ V

REF

´ (V

0.7)

REF -

LEDMAX

R1+R2

´ V

REF

´ (V

0.14)

REF -

=(V

0.6)

FET

LED

TLC5960

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SBVS147 – SEPTEMBER 2010

SETTING THE DC/DC CONVERTER OUTPUT RANGE

The given VLED voltage setting by the iHVM system is shown in Equation 8. VREF is the internal reference voltage

of the dc/dc converter. VHVM is the regulated voltage output from the HVM buffer, which outputs 0.7V initially and

is automatically adjusted by the internal iHVM mechanism.

(8)

The voltage range for VHVM is 0.14V (min) to 1.25V (max) with a typical value of 0.7V. Given these values, the

available VLED range with iHVM is calculated as shown in Equation 9 through Equation 11:

(9)

(10)

(11)

In this case, if VREF = 0.7V, R1 = 200kΩ, R2 = 1kΩ, and R3 = 20kΩ, then the variable range on the dc/dc output,

VLED, is 140V ±6V. The appropriate VLED voltage range can easily be set by selecting the values of R1, R2, and

R3.

BASIC POWER DISSIPATION CONSIDERATIONS

The most critical components for power consumption are the external FETs in the system. These components

are critical because the forward voltage variance of all the LED strings are concentrated in these components.

For optimal system performance with the appropriate thermal dissipation, the LED selection and iHVM VLED

range must be set accordingly. The iHVM VLED range should be determined from the VLED worst-case variance

without turning off the LED strings. Here, the FET power dissipation rating (PFET) is defined as shown in

Equation 12:

(12)

If the FET is supposed to tolerate 1W, then the VD should be 10.6V under worst-case conditions with a 100mA

ILED setting.

FLEXIBLE CONFIGURATIONS OF THE iHVM

The TLC5960 provides flexible configurations for the dc/dc converter and LED driver channel count. By default,

the TLC5960 has four PWM inputs, eight LED strings, and four dc/dc converter control outputs (2CH mode). In

this case, the best thermal capability is provided with dc/dc peripherals overhead; see Figure 24. The TLC5960

provides drain node voltage control within the range of 1.6V to 2.6V on at least four out of eight channels. The

upper-right side of Figure 24 illustrates this four-channel control capability that automatically minimizes the FET

power dissipation.

The TLC5960 integrates two other configuration modes: 4CH HVM mode and 8CH HVM mode. In these modes,

the TLC5960 is capable of controlling the headroom voltage range in two out of the eight channels, or one out of

eight channels. Also in these modes, the total VD variance that is coming from the LED VF variance is much

larger than in 2CH mode, as shown in the upper-right side of Figure 25. If the LED components are not

appropriately selected, only one LED string is well-controlled within the HVM range of 1.6V to 2.6V. It is more

important to have a better selection criteria on LEDs instead of than less peripheral overhead in order to better

handle the thermal rating.

Application of voltage greater than 7.0V on CTRL2 drives the TLC5960 HVM mode into 4CH mode. CTRL1 and

CTRL3 PWM inputs are used to control Ch1, Ch2, Ch5, Ch6 and Ch3, Ch4, Ch7, Ch8, respectively. Two HVM

outputs are used to control two dc/dc converter output voltages. If voltage greater than 7.0V is placed on both

CTRL2 and CTRL4, the TLC5960 HVM logic goes into 8CH mode. CTRL1 controls all of the LED string outputs

and the HVM output is consolidated into one HVM1 output. Table 3 summarizes the HVM mode configurations.

The applicable voltage on CTRL2 and CTRL4 is defined up to the VIN level. It is recommended to tie CTRL2

and/or CTRL4 to VIN through a 10k

Ω resistor.

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