IW3612 Datasheet, PDF (13/18 Pages) Dialog Semiconductor – AC/DC Digital Power Controller for Dimmable LED Drivers

iW3612
AC/DC Digital Power Controller for
Dimmable LED Drivers
current multiplied by the I_SENSE sense resistor is greater than
V_OCP over-current protection engages and the IC immediately
turns off the gate drive until the next cycle. The output driver
100
continues to send out switching pulses, but the IC will
80
immediately turn off the gate drive if the OCP threshold is
reached again.
60
If the I_SENSE sense resistor is shorted there is a potential
danger of the over-current condition not being detected.
Thus the IC is designed to detect this sense-resistor-short
fault after the start up, and shutdown immediately. The V_CC
will be discharged since the IC remains biased. In order
to prevent overcharging the output voltage, the iW3612
employs an extended discharge time before restart, similar
to the discharge time described in section 9.10.
9.12 Over Temperature Protection
If an NTC thermistor is connected from the V_T pin to GND
then, the iW3612 is able to detect and protect against an
over temperature event (OTP).
The iW3612 provides a current (I_VT) to the V_T pin and detects
the voltage on the pin. Based on this voltage the iW3612
can monitor the temperature on the NTC thermistor. As the
V_T pin voltage reduces, the iW3612 reduces the amount of
chopping and the output current according to Figure 9.12.
There is a hysteresis of 84 mV on V_T pin voltage for each
power limiting step.
100
80
60
40
20
40
20
00.0
0.2
0.4
0.6
0.8
1.0
V SH-TH
V P-LIM(LO)
V P-LIM(HI)
V_T Pin Voltage
Figure 9.13 : V_T Pin Voltage vs. % of Nominal Output Current
V_T from 0.0V to 1.0V
When the V_T pin voltage reaches V_P-LIM(HI) the output current
begins to reduce as shown in Figure 9.12. At V_P-LIM(LO) the
output current reduces to 1%. The device can be placed
in shutdown mode by pulling the V_T pin to ground or below
V_SH-TH.
9.13 Thermal Design
The iW3612 is typically installed inside a small enclosure,
where space and air volumes are constrained. Under these
circumstances θ_JA (thermal resistance, junction to ambient)
measurements do not provide useful information for this
type of application. Instead we have provided ψ_JB which
estimates the increase in die junction temperature relative to
the PCB surface temperature. Figure 9.14 shows the PCB
surface temperature is measured at the IC’s GND pin pad.
Thermal Epoxy
Artic Silver
Copper Thermal Pad
Under Package
Exposed
Die Pad
J
ψ_JB
B
IC Die
PCB Top Copper Trace
GND pin
00.0
0.2
0.4
0.6
0.8
1.0
V SH-TH
V P-LIM(LO)
V P-LIM(HI)
V_T Pin Voltage
Printed Circuit Board
Printed Circuit Board
Thermal Vias
Connect top thermal pad
to bottom copper
PCB Bottom Copper Trace
Figure 9.12 : V_T Pin Voltage vs. % of Nominal Output Current
V_T from 1.0V to 0.0V
Figure 9.14 : Ways to Improve Thermal Resistance
Using ψ_JB the junction temperature (T_J) of the IC can be
found using the equation below.
T=_J T_B + P_H ⋅ ψ_JB
(9.13)
where, T_B is the PCB surface temperature and P_H is the
power applied to the chip or the product of V_CC and I_CCQ.
The iW3612 uses an exposed pad package to reduce the
thermal resistance of the package. The exposed pad can
Rev. 1.6
iW3612

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