NCV8853

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8

THEORY OF OPERATION

Oscillator

S

R

Q

+

Slope

Compensation

NCV885300

VIN

ISNS

Current Information

Voltage Error

Vout

GDRV

Compensation

CO

L

CSA

VEA

PWM

COMPARATOR

GATE

DRIVE

-

+

Figure 13. Current Mode Control Schematic

COMP

FB

−

+

VREF

Current Mode Control

The NCV8853 SMPS incorporates a current mode control

scheme, in which the PWM ramp signal is derived from the

power switch current. This ramp signal is compared to the

output of the error amplifier to control the on−time of the

power switch. The oscillator is used as a fixed−frequency

clock to ensure a constant operational frequency. The

resulting control scheme features several advantages over

conventional voltage mode control. First, derived from the

resistor in the power path, the signal responds immediately

to line voltage changes. This eliminates the delay caused by

the output filter and error amplifier, which is commonly

found in voltage mode controllers. The second benefit

comes from inherent pulse−by−pulse current limiting by

merely clamping the peak switching current. Finally, since

current mode commands an output current rather than

voltage, the filter offers only a single pole to the feedback

loop. This allows for a simpler compensation.

The NCV8853 also includes a slope compensation

scheme in which a fixed ramp generated by the oscillator is

added to the current ramp. A proper slope rate is provided to

improve circuit stability without sacrificing the advantages

of current mode control.

Overcurrent Protection

The NCV8853 features two current limit protections:

peak current mode and overcurrent hiccup mode. When the

current sense amplifier detects a voltage above the peak

leading edge blanking time, the peak current limit causes the

power switch to turn off for the remainder of the cycle. Set

If the voltage across the current sense resistor exceeds the

overcurrent threshold voltage the part enters overcurrent

hiccup mode. The part will remain off for the hiccup time

and then go through the power on reset procedure.

Short Circuit Hiccup Protection

When the output voltage falls below the short circuit trip

voltage the part enters short circuit latch off. When a short

is detected the NCV8853 disables the outputs and attempts

to re−enable the outputs after the short circuit hiccup time.

The part remains off for the hiccup time and then goes

through the power on reset procedure. If the short has been

removed then the output stage re−enables and operates

normally; however, if the short is still present the cycle

begins again. Internal heat dissipation is kept to a minimum

as current will only flow during the reset time of the

protection circuitry. The hiccup mode is continuous until the

short is removed.