Design guide for LED Lighting Controller XC9401(5/9)

5.Operational explanation

This section explains the control method and operation states of the XC9401 series.
To select external components, refer to section 6 for non-isolated circuits, or section 7 for isolated circuits.

5-1. Normal operation

5-1-1. Current control method and input voltage / input current

The XC9401 series adjusts the LED current by comparing the VSINE pin voltage or IC internal reference voltage to the ISEN pin voltage, which results from conversion of the coil current or transformer primary coil current to a voltage. The operation of the coil current and LED current in each type using a non-isolated circuit is described below.

A type: Supports a high power factor

The A type compares the VSINE pin voltage to the ISEN pin voltage to control the peak current of the coil so that it follows the VSINE pin voltage.
The voltage after full wave rectification is resistance-divided and a voltage in phase with the input voltage (sine wave) is input into the VSINE pin. The voltage input from the VSINE pin is multiplied by 0.2783 inside the IC and compared in the comparator (PWMCMP) to the ISEN pin voltage, which monitors the peak current of the coil due to switching. When the ISEN pin voltage is higher than the comparison voltage, switching is stopped and the peak current of the coil becomes in phase with the VSINE pin voltage, so that the input voltage and input current are in phase and a high power factor can be achieved.

Fig.10 XC9401 A type Non-isolated circuit and operation waveforms

B type: Constant current control

The B type controls the peak current of the coil by comparing the ISEN pin voltage to a voltage of 0.343V (typ.) obtained by multiplying the reference voltage in the IC by 0.2783. This makes the peak current of the coil constant regardless of the input voltage, enabling good line regulation characteristics as a LED current to be obtained.

Fig.11 XC9401 B type Non-isolated circuit and operation waveforms

5-1-2. Off-time fixed control and calculation of on time/off time

The XC9401 series fixes the off time of the external power MOSFET to 6.0μs (typ.) and controls the current that flows to the power MOSFET. During switching on operation, off occurs when the peak current of the coil or transformer primary coil is detected, and the next on operation starts after the fixed off time elapses. This sequence is repeated continuously.

Because the peak current of the coil is monitored by means of the ISEN pin voltage, the on time depends on the slope of the coil current or transformer primary coil current (which depends on the inductance value and voltage Vrec after full wave rectification during switching) and the comparator (PWMCMP) comparison voltage. Particularly in the B type, which is the source of the comparison voltage, changes continuously, and the on time changes accordingly. As a result, the switching frequency is dispersed rather than becoming a specific frequency, which makes it possible to reduce the EMI level.

The method of calculating on time / off time and the current waveform during operation are different in a non-isolated circuit and an isolated circuit, as explained the next section.

Non-isolated Circuit

During operation in discontinuous mode in a non-isolated circuit, the fixed off time is maintained until the next on even if the coil current becomes 0A (Fig. 12). In continuous mode, on occurs after the fixed off time when the coil current is 0A or higher (Fig. 12).
The on time tON and off time tOFF' of discontinuous mode are given by Equations (1) and (2).
To stabilize IC operation, the minimum on time is set internally to 0.2μs.

Fig.12 Coil Current in discontinuous mode

Fig.13 Coil Current in continuous mode

VLED LED voltage
Vrec(t) Voltage after full wave rectification at time t
ΔIL Coil current amplitude
VF Forward voltage of flywheel diode
L Coil inductance value

Reference calculation results can be calculated in the separate calculation file.

Isolated Circuit

In an isolated circuit, current flows to the transformer primary coil while the external power MOSFET is on, and current flows in the secondary coil while the MOSFET is off. (Fig. 14, Fig. 15)
In discontinuous mode, the fixed off time is maintained until the next on even if the transformer secondary coil circuit becomes 0A. In continuous mode, on occurs when the transformer secondary coil current is 0A or higher after the fixed off time.
The on time tON and off time tOFF' of discontinuous mode are given by Equations (3) and (4). To stabilize IC operation, the minimum on time is set internally to 0.2μs.

Fig.14 transformer Current in discontinuous mode

Fig.15 transformer Current in continuous mode

VLED LED voltage
Vrec(t) Voltage after full wave rectification at time t
ΔIL Transformer primary current amplitude
VF Forward voltage of rectification diode
L Inductance value of transformer
N1 Number of windings of transformer primary coil
N2 Number of windings of transformer secondary coil

Reference calculation results can be calculated in the separate calculation file.

5-2. Startup

To allow PWM dimming to be performed from the EN/DIM pin, the XC9401 does not have a soft start function.
When the IC starts, the ISEN pin voltage is monitored by means of the external RSEN resistance and the peak current in the coil or transformer primary coil is controlled, so a rush current higher than the set current never flows.

EN Startup

When a voltage higher than the UVLO release voltage is applied to the VDD pin, the IC can be started by inputting a signal higher than the H level voltage into the EN/DIM pin. Normal operation starts following a delay of 140 μs (typ.) after the EN/DIM pin reaches the H level voltage. (Fig. 16)

Fig.16 EN Startup

AC Startup (VDD=EN)

Following AC power supply input, CVDD is charged through RVDD from the voltage Vrec that has been smoothed after full wave rectification, and this raises the voltage of the VDD pin. When the UVLO release voltage 7.5V (typ.) is reached, UVLO is released and normal operation resumes. (Fig. 17)

Fig.17 AC Startup

An approximate value for the time from AC power supply input until normal operation can be calculated using Equation (5).

Fig.18 VDD circuit diagram

RVDD Refer to fig.18.
CVDD Refer to fig.18.
VUVLOR UVLO release voltage 7.5V (typ.)
ISTB Stand-by Current 225μA (typ.)
Vrms Input RMS Voltage (ex.220V)

5-3. Standby state

The internal circuitry of the IC is put in the standby state by inputting a voltage lower than the L level voltage into the EN/DIM pin. In the standby state, switching stops but the internal circuitry of the IC continues to operate. This turns off the LED and reduces power consumption.

5-4. Dimming

By inputting the PWM signal into the EN/DIM pin, on/off of the GATE output is controlled in synchronization with the PWM signal to perform PWM dimming. As a guideline, the frequency used for PWM dimming should be about 500Hz to 1kHz. The GATE output that drives the external power MOSFET outputs a signal 140μs after the EN/DIM pin voltage reaches the H level voltage, and thus a minimum on duty of 140μs or longer is required, and the maximum on duty less than 100% duty is one cycle minus 140μs.

Fig.19 Timing of PWM Dimming (XC9401 B type)

5-5. Protective Functions

The XC9401 series has four protective functions: over-current protection, thermal shutdown, UVLO, and VDD over-voltage protection.

5-5-1. Over Current Limit

When the switching current of the external power MOSFET is in the over-current state and the ISEN pin voltage reaches 0.7V (typ.), L level voltage is output to the GATE pin and the external power MOSFET is turned off. In addition, the off time is extended from the normal 6.0μs to 140μs. When the ISEN pin voltage falls below 0.7V (typ.) after the extended off time, normal operation resumes.

When a short circuit occurs between LEDs in a non-isolated circuit, the current slope of the coil (L2) during the off time becomes smaller than the slope during normal switching, and in an off time of 6.0μs, sufficient discharge cannot take place. The external power MOSFET Q1 always turns on during the minimum on time, so the coil current gradually increases. The ISEN pin voltage becomes higher in synchronization with the increase of coil current, and when the ISEN pin voltage reaches 0.7V, the off time is extended to about 140μs. (Fig. 20)

Fig.20 Over current limit(operation when a short circuit occurs between the LEDs in non-isolated circuit)

5-5-2. Thermal Shutdown

To protect the IC from thermal destruction, thermal shutdown activates when the chip temperature reaches 150°C(typ.), and the GATE pin voltage is forcibly put in the “L” state to reduce the power supplied to the LED. When the chip temperature drops to 130°C(typ.), normal operation automatically resumes.

5-5-3. UVLO

When the VDD pin voltage falls below the UVLO detection voltage (VUVLO), the GATE pin voltage is forcibly put in the “L” state to prevent the output of false pulses. When the VDD pin voltage rises above the UVLO release voltage (VUVLOR), normal operation resumes.
When UVLO is detected, switching is stopped but the internal circuitry of the IC continues to operate.

5-5-4. VDD Over-voltage Protection

This function prevents IC destruction when over-voltage is input into the VDD pin in the standby state and other states. When the VDD pin voltage exceeds the VDD over-voltage detection voltage (VOVP), the capacitor CVDD between the VDD pin and GND pin is discharged through the internal IC resistance between the VDD pin and GND pin (Fig. 21). At this time, the GATE pin voltage is forcibly put in the “L” state. When the VDD pin voltage drops below the VDD over-voltage release voltage (VOVPR), normal operation resumes. (Fig. 22)
In a configuration where a transformer is used in the power supply to the IC (Fig. 21), the above operation (Fig. 22) actually takes place when the IC goes into the standby state.

Fig.21 VDD power supply circuit using a transformer

Fig.22 VDD Over-voltage protection operation

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