NCP1653_09 datasheet(13/19 Pages) ONSEMI

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NCP1653, NCP1653A

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becomes its maximum value and generates zero on time t1.

Then, Vout decreases and the minimum can be Vout = Vin in

a boost converter. Going down to Vin, Vout automatically

enters the previous two regions (i.e., follower boost region

or constant output voltage region) and hence output voltage

Vout cannot reach input voltage Vin as long as the NCP1653

provides a duty ratio for the operation of the boost

converter.

In conclusion, the NCP1653 circuit operates in one of the

following conditions:

Constant output voltage mode: The output voltage is

regulated around the range between 96% and 100% of RFB

× Iref. The output voltage is described in (eq.16). Its

behavior is similar to a follower boost.

Follower boost mode: The output voltage is regulated

under 96% of RFB × Iref and Icontrol = Icontrol(max) = Iref/2 =

100

mA. The output voltage is described in (eq.15).

Overvoltage Protection (OVP)

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

reference current Iref (i.e., Vout > 107% RFB × Iref ), the

Drive Output (Pin 7) of the device goes low for protection.

The circuit automatically resumes operation when the

feedback current becomes lower than 107% of the

reference current Iref.

The maximum OVP threshold is limited to 230

mA which

corresponds to 230

mA × 1.92 MW + 2.5 V = 444.1 V when

RFB = 1.92 MW (680 kW + 680 kW + 560 kW) and

VFB1 = 2.5 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)

Figure 35. Undervoltage Protection

8% I

12% I

Shutdown

Operating

ref

ICC

ICC2

Istdn

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

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

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

consumes less than 50

mA. The device automatically starts

operation when the output voltage goes above 12% of the

nominal regulation level. In normal situation of boost

converter configuration, the output voltage Vout is always

greater than the input voltage Vin and the feedback current

IFB is always greater than 8% and 12% of the nominal level

to enable the NCP1653 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.

Soft−Start

The device provides no output (or no duty ratio) when the

Vcontrol (Pin 2) voltage is zero (i.e., Vcontrol = 0 V). An

external capacitor Ccontrol connected to the Vcontrol pin

provides a gradually increment of the Vcontrol voltage (or

the duty ratio) in the startup and hence provides a soft−start

feature.

Current Sense

The device senses the inductor current IL by the current

sense scheme in Figure 36. The device maintains the

voltage at the CS pin (Pin 4) to be zero voltage (i.e.,

VS ≈ 0 V) so that (eq.10) can be formulated.

Figure 36. Current Sensing

NCP1653

Gnd

−

RCS

This scheme has the advantage of the minimum number

of components for current sensing and the inrush current

limitation by the resistor RCS. Hence, the sense current IS

represents the inductor current IL and will be used in the

PFC duty modulation to generate the multiplier voltage

VM, Overpower Limitation (OPL), and overcurrent

protection.

Overcurrent Protection (OCP)

Overcurrent protection is reached when IS is larger than

IS(OCP) (200 mA typical). The offset voltage of the CS pin

is typical 10 mV and it is neglected in the calculation.

Hence, the maximum OCP inductor current threshold

IL(OCP) is obtained in (eq.15).

(eq.18)

IL(OCP) +

RSIS(OCP)

RCS

200 mA

When overcurrent protection threshold is reached, the

Drive Output (Pin 7) of the device goes low. The device

automatically resumes operation when the inductor current

goes below the threshold.

Input Voltage Sense

The device senses the RMS input voltage Vac by the

sensing scheme in Figure 37. The internal current mirror is

with a typical 4 V offset voltage at its input so that the

current Ivac can be derived in (eq.9). An external capacitor

Cvac is to maintain the In pin (Pin 3) voltage in the

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