NCV8870

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7

TYPICAL PERFORMANCE CHARACTERISTICS

Figure 8. Enable Pulldown Current vs. Voltage

Figure 9. Enable Pulldown Current vs.

Temperature

012

3

4

56

−40

10

60

110

160

0

1

2

3

4

5

7

6

5.0

5.5

6.0

6.5

7.0

7.5

8.0

THEORY OF OPERATION

Figure 10. Current Mode Control Schematic

Oscillator

Q

S

R

NCV8870

Voltage Error

VEA

CSA

PWM Comparator

Gate

Drive

Compensation

L

GDRV

−

+

+

−

+

−

+

Current Mode Control

The NCV8870 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 directly

from the inductor, the ramp signal responds immediately to

line voltage changes. This eliminates the delay caused by the

output filter and the 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 NCV8870 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.