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NCP1601A

Compact Fixed Frequency Discontinuous or Critical Conduction Voltage Mode Power Factor Correction Controller

ON Semiconductor 

NCP1601A, NCP1601B

voltage is higher based on the regulation block

characteristic in Figure 31. On the other hand, the Vcontrol

in the low Vac condition is much higher than the high Vac

condition. In order to not over−design the circuit in the

application, the Vcontrol in the low Vac condition is usually

very closed to Vcontrol(max). It makes the output voltage be

almost 96% of the nominal value of RFB × Iref in low Vac

condition while the output voltage is almost 100% of the

nominal value RFB × Iref in high Vac condition.

The feedback resistor RFB consists of two or three high

precision resistors in order to set the nominal Vout precisely

and safety purpose.

The regulation block output Vreg is connected to control

voltage Vcontrol through an internal resistor Rcontrol

(300 kW typical) for the low−pass filter in Figure 30. The

Vcontrol and the time information of zero current are

collected in the Vcontrol processing circuit to generate Vton

which is then compared to a ramp signal to generate the

MOSFET on time t1 for power factor correction.

Overvoltage Protection (OVP)

When the feedback current IFB is higher than 107% of the

reference current Iref (i.e., the output voltage Vout is higher

than 107% of its nominal value), the Drive Output pin

(Pin 7) of the device goes low for protection and the switch

of the Vcontrol processing circuit is kept off. The circuit

automatically resumes operation when the output voltage

is lower than 107%.

The maximum OVP threshold is limited to 225 mA which

corresponds to 225 mA × 1.95 MW + 5 V = 443.75 V when

RFB = 1.95 MW (1.8 MW + 150 kW) and VFB1 = 5 V (for

the worst case referring to Figure 11). Hence, it is generally

recommended to use 450 V rating output capacitor to allow

some design margin.

Undervoltage Protection (UVP)

When the feedback current IFB is lower than 8% of the

reference current Iref (i.e., the output voltage Vout is lower

than 8% of its nominal value), the device is shut down and

consumes lower than 50 mA. In normal situation of boost

converter configuration, the output voltage Vout is always

higher than the input voltage Vin and the feedback current

IFB is always higher than 8% of the reference current Iref.

It enables the NCP1601 to operate. Hence, UVP happens

when the output voltage is abnormally undervoltage, the

FB pin (Pin 1) is opened, or the FB pin (Pin 1) is manually

pulled low.

Current Sense

The device senses the inductor current IL by the current

sense scheme in Figure 32. This scheme has the advantages

of: (1) the inrush current limitation by the resistor RCS, and

(2) the overcurrent protection and zero current detection

implemented in the same pin.

IL

RS

IS

RCS IL

CS NCP1601

+

VS

Gnd

−

Figure 32. Current Sensing

Inductor current IL passes through RCS and creates a

negative voltage. This voltage is measured by a current IS

flowing out of the CS pin (Pin 4). The CS pin has an offset

voltage VS. This offset voltage is studied in the setting of

zero inductor current IL(ZCD) and the maximum inductor

current IL(OCP) (i.e., overcurrent protection threshold). A

typical variation of offset voltage VS versus sense current

IS is shown in Figure 15. Higher the value of the offset

voltage at low current region creates lower the zero current

threshold for better accuracy. Based on Figure 32, (eq.13)

is derived.

VS * RS IS + −RCS IL

(eq.13)

Zero Current Detection (ZCD)

The device recognizes zero inductor current when the CS

pin (Pin 4) sense current IS is lower than IS(ZCD) (14 mA

typical). The offset voltage of the CS pin in this condition

is VS(ZCD) (7.5 mV typical). It is illustrated in Figure 33.

The inductor current IL(ZCD) at the ZCD condition is

derived in (eq.14).

IL(ZCD)

+

RSIS(ZCD) *

RCS

VS(ZCD)

(eq.14)

It is obvious that the IL(ZCD) is not always zero. In order

to make it reasonably close to zero, the settings of RS and

RCS are crucial.

VS

RS > RS(ZCD)

Operating ZCD point

RS = RS(ZCD)

VS(ZCD)

Ideal ZCD point

IS

IS(ZCD)

Figure 33. CS Pin Characteristic when IL = 0

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