HV9801ANG-G(2011) 데이터 시트보기 (PDF) - Supertex Inc

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HV9801ANG-G
(Rev.:2011)

Switch Dimmable LED Driver

Supertex Inc 

HV9801A

Duty Cycle, Off-time, On-time, Inductor

Duty cycle

The duty cycle (D) is related to the load voltage (VLED) and

input voltage (VBUS) by the simple relation:

VOUT = (D) • (VIN )

VLED = (D) • (VBUS )

Off-time

The HV9801A employs constant off-time control. Operation

with constant off-time suppresses subharmonic oscillation.

Switching period and switching frequency are related to on-

time and off-time as follows:

TSW = (TON + TOFF )

FSW = 1 / TSW

On-time is related to off-time and duty cycle as follows:

D = (TON ) / (TON + TOFF )

With a given TOFF, the HV9801A dynamically adjusts TON to

regulate the LED current. Specifically, TON adapts to the duty

cycle associated with given VBUS and VLED.

Inductance can be calculated from the current drop during

off-time:

(L) • (ΔI) = (V) • (ΔT)

(L) • (PPR) • (ILED ) = (VLED ) • (TOFF )

For instance, 30% PPR on 350mA average current equates

to 105mA ripple, which together with 5µs off-time, and 30V

LED string voltage corresponds to 1.43mH inductance.

A design with 30V LED voltage and with 150V bus voltage

corresponds to 20% duty cycle; respectively, with a 120V

bus voltage to 25% duty cycle. 20% duty cycle corresponds

to 1.25µs on-time, 25% duty cycle corresponds to 1.67µs.

Hence, the switching frequency is 167kHz at 150V bus volt-

age and 150kHz at 120V bus voltage.

Maximum Duty Cycle

Duty cycle should be limited to the specified maximum

(80%). Accordingly, the LED string voltage and the bus volt-

age are limited to the same ratio. Operation at a larger duty

cycle results in an LED current lower than programmed.

Minimum Duty Cycle

Duty cycle is limited on the low side by the minimum on-time

specification (760ns). Operation at a smaller on-time causes

the LED current to exceed the programmed value.

Off-time Programming

Off-time is programmed by the RT resistor:

LED string voltage can not be made arbitrarily low. Minimum

LED voltage can be determined from the following:

TOFF = (A) • (RRT) + B

DMIN = (TONX ) / (TOFF + TONX )

where A = 40ps / Ω and B = 300ns

VLED = (DMIN ) • (VBUS )

For instance, a 200kΩ resistor corresponds to 8.3μs off-time.

An acceptable range for RT is 30kΩ to 1MΩ, corresponding

to an off-time range between 1.5µs and 40.3µs.

Inductor

The converter should operate in continuous conduction

mode. As such, the inductor current should not fall to zero

within a switching cycle, and inductor current ripple should

be sized accordingly.

A common choice for peak-to-peak Inductor current ripple

(PPR) is 30 to 40% of nominal LED current.

For instance, with 5µs off-time, the duty cycle should be kept

above 13%. Such a duty cycle corresponds to an LED string

voltage of 19.5V at 150V bus voltage.

A design requiring a lower LED string voltage requires

change to a longer off-time.

Short Circuit Protection

A rise of the LED current sense signal above 440mV (176%

of nominal) trips the short circuit comparator thereby caus-

ing the converter to switch to hiccup mode. In hiccup mode,

off-time is lengthened to about 750µs to allow the inductor

current to drop to a safe level.

Supertex inc. ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com

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