HT7L5600
primary side MOSFET will be limited to VOCPL/RCS,
thus maintaining a lower current to protect the power
components. If the detected voltage is equal to or
higher than 0.4V, the device will set the OCP detection
voltage to VOCPH (1.55V). When this happens the peak
current will be limited to VOCPH/RCS, which should be
higher than the peak current under normal operation to
avoid the LED current falling below the set value for
normal operation.
Over Voltage Protection – OVP on VCC
In order to prevent PWM controller damage, the
device includes an OVP function for VCC. Should the
VCC voltage exceed the OVP threshold voltage of 24V,
the PWM controller will cease operation immediately
When the VCC voltage falls below the UVLO off
level, the controller will reset.
.
LED Open Protection – ZCD OVP
DRV
VZCD
ZCD
The ZCD pin voltage is set by a resistor divider RTOP
(top resistor), RBOT (bottom resistor) and an auxiliary
winding due to the coupling polarity between the
auxiliary winding and the secondary winding of the
transformer. Once the ZCD voltage exceeds 3.2V
after a blanking time about 1us to allow the leakage
inductance ringing to be fully damped, ZCD OVP is
triggered. The device will then stop switching but it
can be reset by re-starting the voltage on the VCC pin.
The OVP voltage can be adjusted by the equation:
1μs
Figure 11. ZCD Signal Sensing
As shown in Figure 12, after the AC power is turned
on under normal operation, the VCC voltage will
be charged by the start-up resistor until the UVLO
function is on. After this happens the device can then
be activated. The initial value of VZCD is lower than
0.4V and the OCP detection voltage on the CS pin is
VOCPL. When the VZCD voltage rises to a value equal
to or greater than 0.4V, the device determines that
the LED is not in a short circuit condition and sets
the OCP detection voltage to VOCPH. When an LED
short circuit condition occurs under steady normal
operation, the VLED voltage tends to 0V, and the VCC
voltage drops. It should be noted that although the
VZCD voltage is lower than 0.4V, the OCP detection
voltage set as VOCPH will remain unchanged. When the
UVLO On/Off restarts and the VZCD voltage remains
lower than 0.4V, the device will detect an LED
short circuit condition. When this happens the OCP
detection voltage will maintain the initial setting of
VOCPL, and consequently the peak current of VOCPL/RCS
will be lower than the normal operating current. Now
the device will continuously enter the UVLO On/
Off restart state. In such cases, the components can
be protected from being damaged by the continuous
flow of high current. When the LED short circuit
condition ceases to exist, the VZCD voltage will rise to
a value equal to or greater than 0.4V. The device will
then determine that the LED is not in a short circuit
condition and the OCP detection voltage on the CS
pin will resume to VOCPH for normal operation.
R
VOUT _OVP = 3.2×(1+ TOP )×
RBOT
NS
NA
+VD
Where NS is the secondary winding, NA is the
auxiliary winding and VD is the forward bias voltage
of the secondary diode.
Over Current Protection – OCP
The HT7L5600 includes a CS pin over current protection
function. An internal circuit detects the current level
and should the current be larger than the over current
protection threshold level, the gate output will then be
fixed to a low level.
LED Short Protection – SCP
When an LED short condition occurs, the SCP func-
tion can prevent power components from being dam-
aged due to high currents. As shown in Figure 11, the
device determines whether the LED is in a short cir-
cuit condition by detecting the voltage on the ZCD pin
when the DRV output is low. If the detected voltage
is lower than 0.4V, the device will set the low OCP
detection voltage on the CS pin to VOCPL (0.4V). When
this happens, the peak current flowing through the
Rev. 1.30
7
September 26, 2018
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