3
AN-H64
Supertex inc.
●
1235 Bordeaux Drive, Sunnyvale, CA 94089
●
Tel: 408-222-8888
●
www.supertex.com
( (
Short-Circuit Protection
Both the HV
9910B and HV9961 are characterized by a min
-
imum on-time of the GATE output. This minimum on-time
includes the leading-edge blanking delay and the current-
sense comparator propagation delay. The minimum on-
time is 0.47µs(max) for the HV9910B and 1.0µs(max) for
the HV9961. When a short circuit is applied at the output of
the buck converter, the only voltage available to reset the
magnetic flux in the inductor during t
OFF
is the rectifier di
-
ode voltage drop. When the converter keeps switching at
the same frequency rate this may not be enough. Therefore,
the inductor current will keep rising every switching cycle.
(See Figure 4.)
The HV9961 is protecting the LED driver from such “stair-
case” saturation of the inductor by introducing a second
threshold I
LIM
= 0.44V/R
CS
. When this threshold is reached,
the GATE output becomes disabled for 400µs, thus letting
the inductor current ramp down to a safe level.
Constant-Frequency and Constant Off-Time
Operating Modes
The HV9
910B can be configured for operating in either
switching mode. When RT is connected to GND, it maintains
a constant switching frequency. Wiring RT to GATE yields a
fixed t
OFF
mode. The corresponding timing equations are:
t
OSC
= 40pF • R
T
+ 0.88µs
(5a)
t
OFF
= 40pF • R
T
+ 0.88µs
(5b)
where t
OSC
is the switching period with RT wired to GND, and
t
OFF
is the off-time with RT connected to GATE.
The HV9961 does not support the fixed frequency mode.
Moreover, the R
T
resistor must be wired to GND in all cases.
Therefore, the HV9961 cannot be used as a direct drop-in
replacement in the applications of the HV9910B wired for
the fixed t
OFF
operation, and a layout change is required. The
HV9961 t
OFF
is given by:
t
OFF
= 40pF • R
T
+ 0.3µs
(5c)
If the HV9910B is wired for the fixed frequency operation,
the conversion to the HV9961 will merely require the R
T
re-
sistor value change. Since t
OFF
= (1-V
O
/ V
IN
) • t
OSC
, Equations
5a and 5c can be solved for the new R
T
value:
R
T(HV9961)
= 1 -
V
O
• (R
T(HV9910B)
+ 22kΩ) - 7.5kΩ
(6)
V
IN
Duty Cycle Range
The duty cycle is determined by the equation D = t
ON
/t
OSC
= t
ON
/(t
OFF
+ t
ON
). Both the HV9910B and the HV9961 have
their minimum duty cycle D
min
limited by the minimum on-
time. However, with the HV9961 the guaranteed ±3% accu
-
racy of the LED current can only be achieved with the duty
cycle D
min
>0.08~0.1.
The maximum duty cycle of the HV9910B operating with
the fixed frequency is limited to D
max
= 0.5. Exceeding D =
0.5 with this operating mode causes sub-harmonic oscilla-
tion at ½ of the switching frequency. When the HV9910B is
operated with fixed t
OFF
, there is no theoretical limit of D
max
.
However, due to parasitic resistances in the circuit and large
switching frequency variation, it is not recommended that a
D
max
= 0.8 be exceeded with this operating mode.
With the HV9961, regulation of the average inductor current
is limited to D
max
≤ 0.75. When D = 0.75 (125
o
C) or D = 0.8
(105
o
C) is exceeded, the functionality of the HV9961 will be
-
gin approaching that of the HV9910B, and the LED current
will drop accordingly.
Figure 4. Effect of the output short circuit on the inductor current.
HV9910B
HV9961
SHORT
400µs
I
LIM