V

(V)

LED

V

INIT

V

OPT

Time(s)

Time(s)

V

(V)

LED

V

INIT

V

OPT

(a) VLEDTypicalTransientResponseoniHVMsystem

(b) VeryLowTransientLossoniHVMsystemwithFastSwitching

t

F

t

R

t

F

Time(2 s/div)

m

I

(0mA)

LED

I

(50mA)

LED

2 s

m

V

(150V 200mV))

±

LED

TLC5960

SBVS147 – SEPTEMBER 2010

www.ti.com

Figure 32 shows the iHVM transient response. Only the first cycle of the HVM cycle is needed to set the

appropriate voltage for the VLED corresponding to the LED strings total VF. Therefore, the first tF should be a

similar transient with conventional system; however, from the next cycle, the intelligent iHVM mechanism reuses

the once-settled value. As shown in the results, the second falling edge can be much faster than the first cycle,

and almost zero transient power consumption is achievable in Figure 32(a).

Figure 32. Transient Response of the iHVM-Based System

In addition to this power saving, faster LED switching speeds can be set up to 250kHz. The transient response in

this case is shown in Figure 32(b). The power-saving iHVM mechanism is designed to work perfectly with that

faster switching speed, as well.

Figure 33 shows the application evaluation results of an iHVM system. As the dc/dc portion, the TPS40210

SEPIC configuration (supply voltage = 170V, VLED ~150V) is employed. 48 LEDs are series connected and RS is

set to 12

Ω in order to achieve a 50mA constant current. The iHVM system works well to keep the once-settled

VLED voltage well-regulated with a constant current value.

Figure 33. Switching Characteristics for 1% PWM Dimming on 240Hz

28

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