HV9801A
Inductance can be calculated from the current drop during
off-time:
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:
(L) • (ΔI) = (V) • (ΔT)
(L) • (PPR) • (ILED ) = (VLED ) • (TOFF
)
VOUT = (D) • (VIN )
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.
VLED = (D) • (VBUS
Off-time
)
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.
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:
T
SW = (TON + TOFF
FSW = 1 / TSW
On-time is related to off-time and duty cycle as follows:
D = (TON ) / (TON + TOFF
)
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
With a given TOFF, the HV9801A dynamically adjusts TON to
regulate the LED current. Specifically, TON adapts to the duty
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.
cycle associated with given VBUS and VLED
.
Off-time Programming
LED string voltage can not be made arbitrarily low. Minimum
LED voltage can be determined from the following:
Off-time is programmed by the RT resistor:
DMIN = (TONX ) / (TOFF + TONX
VLED = (DMIN ) • (VBUS
)
TOFF = (A) • (RRT) + B
)
where A = 40ps / Ω and B = 300ns
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.
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.
A design requiring a lower LED string voltage requires
change to a longer off-time.
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.
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.
A common choice for peak-to-peak Inductor current ripple
(PPR) is 30 to 40% of nominal LED current.
Doc.# DSFP-HV9801A
B051412
Supertex inc.
5
www.supertex.com
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