HV9912
Preliminary
continuous and a very large output capacitor is required to
reduce the ripple in the LED current. Thus, this capacitor will
have a significant impact on the PWM dimming response.
By turning off the disconnect switch when PWMD goes low,
the output capacitor is prevented from being discharged and
thus the PWM dimming response of the boost converter is
greatly improved.
Note that in case of continuous conduction mode boost con-
verters, disconnecting the capacitor might cause a sudden
spike in the capacitor voltage as the energy in the inductor
is dumped into the capacitor. This increase in the capacitor
voltage might cause the OVP comparator to trip if the OVP
point is set too close to the maximum operating voltage.
Thus, either the capacitor has to larger to absorb this energy
without increasing the capacitor voltage significantly or the
OVP set point has to be increased.
False Triggering of the Short Circuit Comparator During
PWM Dimming
During PWM dimming, the parasitic capacitance of the LED
string causes a spike in the output current when the discon-
nect FET is turned on. With the HV9911, this parasitic spike
in the output current caused the IC to falsely detect an over
current condition and shut down. To prevent this false shut-
down, an R-C filter was used at the FDBK pin to filter this
spike.
To prevent these false triggering in the HV9912, there is a
built-in 500ns blanking network for the short circuit compara-
tor, which eliminates the need for the external R-C low pass
filter. This blanking network is activated when the PWMD
input goes high. Thus, the short circuit comparator will not
see the spike in the LED current during the PWM Dimming
turn-on transition. Once the blanking timer is completed, the
short circuit comparator will start monitoring the output cur-
rent. Thus, the total delay time for detecting a short circuit
will depend on the condition of the PWMD input.
If the output short circuit exists before the PWMD signal
goes high, the total detection time will be:
COMP
the COMP pin is disconnected from the G
M
amplifier and the
GATE and FLT pins are pulled low disabling the LED driver.
When the fault has cleared, a 5μA current source is activated
which pulls the COMP network up to 5V. Once the voltage
at the COMP network reaches 5V, the 5μA sourcing current
is disconnected and a 5μA sinking current is activated which
pulls the COMP pin low. When the voltage at the COMP pin
reaches 1V, the sinking current is disconnected and the Gm
amplifier is reconnected to the COMP pin. The FLT pin goes
high and the GATE pin is now allowed to switch. The closed
loop control then takes over the control of the LED current.
Startup Condition
The startup waveforms are shown in Fig. 2.
Assuming a pole-zero R-C network at the COMP pin (series
combination of RZ and CZ in parallel with C
C
), the start-up
delay time can be approximately computed as
t
delay
≈ t
POR
+ ( C
C
+ C
Z
) x 9V
5µA
(Eqn. 8)
This equation assumes that the voltage drop across RZ can
be neglected compared to the voltage swing at the COMP
pin, which is true in most of the cases (RZ < 100kΩ). The
POR time (t
POR
) for the HV9912 is 10μs.
V
IN
POR
Pull-up
with 5µΑ
Pull-down
with 5µΑ Gm c ontrol
t
detect1
= t
blank,SC(max)
+ t
delay(max)
≈ 700 + 250
≈ 950ns(max)
(Eqn. 6)
5V
If the short circuit occurs when the PWMD signal is already
high, the time to detect will be:
t
detect1
= t
delay(max)
≈ 250ns(max)
(Eqn. 7)
1V
t
POR
FLT
tdelay
Hiccup Timer
The HV9912 reuses the compensation network on the
COMP pin to create a timer which is activated upon startup
or when a detected fault has been cleared. When a fault is
detected (either open circuit or short circuit) or upon startup,
Fig. 2 Waveforms During Startup
8
SUPERTEX [ Supertex, Inc ]
