IW1812 Datasheet, PDF (13/17 Pages) Dialog Semiconductor – Off-Line Digital Green-Mode PWM Controller Integrated with Power BJT and OTP

iW1812
Off-Line Digital Green-Mode PWM Controller
Integrated with Power BJT and OTP
system designs including the pre-load resistor selection should ensure the power supply can stably operate in the
DDPWM mode at the steady-state no-load condition. If the pre-load resistor is too small, the no-load power consumption
will increase; on the other hand, if it is too large, the output voltage may increase and even cause over-voltage since
the switching frequency is fixed at around 1.9kHz. For typical designs, the pre-load resistor is in the range of 5kW to 8kW.
Aside from the appropriate use of pre-load resistor, the iW1812 enjoys a few other features to bring down no-load
power consumption as well. First, the iW1812 implements an intelligent low-power management technique that
achieves ultra-low chip operating current at the no-load, typically less than 400µA. Second, the use of the power
switch of BJT instead of MOSFET requires a lower driving voltage, enabling a low UVLO threshold as low as 4.0V
(typical). The power supply system design can fully utilize this low UVLO feature to have a low Vcc voltage at the
no-load operation in order to minimize the no-load power. In addition, the ultra-low start-up current during the
ramp-up of V_CC towards the start-up threshold V_CC(ST) (see Figure 8.5), allows for the use of high resistance start-up resis-
tors to minimize their loss while still retaining reasonable turn-on time. All together these features ensure that with the
lowest system cost, power supplies built with the iW1812 can achieve less than 30mW no-load power consumption at
230 V_AC input and very tight constant voltage and constant current regulation over the entire operating range including
the no-load operation.
While achieving super-low no-load power consumption, the iW1812 implements innovative proprietary digital control
technology to intelligently detect any load transient events, and achieve fast dynamic load response for both one-time and
repetitive load transients. In particular, for load transients that are demanded in some applications as from absolutely no
load to full load, the iW1812 can still guarantee a fast enough response to meet the most stringent requirements, with
the no-load operating frequency designed at around 1.9kHz.
10.8 Variable Frequency Operation Mode
During each of the switching cycles, the falling edge of V_SENSE is checked. If the falling edge of V_SENSE is not detected,
the off-time is extended until the falling edge of V_SENSE is detected. The maximum transformer reset time allowed is
125μs. When the transformer reset time reaches 125μs, the iW1812 shuts off.
10.9 Internal Loop Compensation
The iW1812 incorporates an internal Digital Error Amplifier with no requirement for external loop compensation. For
a typical power supply design, the loop stability is guaranteed to provide at least 45 degrees of phase margin and
-20dB of gain margin.
10.10 Voltage Protection Features
The secondary maximum output DC voltage is limited by the iW1812. When the V_SENSE signal exceeds the output
OVP threshold at point 1 (as shown in Figure 10.3), the iW1812 shuts down.
The iW1812 protects against input line under-voltage by setting a maximum T_ON time. Since output power is
proportional to the squared V_INT_ON product, for a given output power, the T_ON increases as the V_IN decreases. Thus by
knowing when the maximum T_ON time occurs, the iW1812 detects that the minimum V_IN is reached, and then it shuts
down. The maximum t_ON limit is set to 15.6μs. Also, the iW1812 monitors the voltage on the V_CC pin and when the
voltage on this pin is below UVLO threshold the IC shuts down immediately.When any of these faults is met the IC
remains biased to discharge the V_CC supply. Once V_CC drops below the UVLO threshold, the controller resets itself and
then initiates a new soft-start cycle. The controller continues attempting start-up until the fault condition is removed.
10.11 PCL, OCP and SRS Protection
The peak-current limit (PCL), over-current protection (OCP) and sense-resistor short protection (SRSP) are built-in
features in the iW1812. With the I_SENSE pin the iW1812 is able to monitor the peak primary current. This allows for
cycle-by-cycle peak current control and limit. When the peak primary current multiplied by the I_SENSE resistor is greater
than 1.15V, over-current protection (OCP) is detected and the IC immediately turns off the base driver until the next
cycle. The output driver sends out a switching pulse in the next cycle, and the switching pulse continues if the OCP
threshold is not reached; or, the switching pulse turns off again if the OCP threshold is reached. If the OCP occurs for
Datasheet
Rev. 1.2
26-September-2016
www.dialog-semiconductor.com
13 of 17
© 2016 Dialog Semiconductor