Datasheet
Current Loop
ISENSE and ICOMP functions
Pulse Width Modulator
PWMcycle
PWM
V
RAMP
=
F(V
VCOMP
)
V
ICOMP
t
ON
t
OFF
t
UCC28019A
SLUS828B – DECEMBER 2008 – REVISED APRIL 2009 ..................................................................................................................................................
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The overall system current loop consists of the current averaging amplifier stage, the pulse width modulator
(PWM) stage, the external boost inductor stage and the external current sensing resistor.
The negative polarity signal from the current sense resistor is buffered and inverted at the ISENSE input. The
internal positive signal is then averaged by the current amplifier (g
mi
), whose output is the ICOMP pin. The
voltage on ICOMP is proportional to the average inductor current. An external capacitor to GND is applied to the
ICOMP pin for current loop compensation and current ripple filtering. The gain of the averaging amplifier is
determined by the internal VCOMP voltage. This gain is non-linear to accommodate the world-wide ac-line
voltage range.
ICOMP is connected to 4V internally whenever the device is in a Fault or Standby condition.
The PWM stage compares the ICOMP signal with a periodic ramp to generate a leading-edge-modulated output
signal which is High whenever the ramp voltage exceeds the ICOMP voltage. The slope of the ramp is defined
by a non-linear function of the internal VCOMP voltage.
Figure 26. PWM Generation
The PWM output signal always starts Low at the beginning of the cycle, triggered by the internal clock. The
output stays Low for a minimum off-time, t
OFF_min
, after which the ramp rises linearly to intersect the ICOMP
voltage. The ramp-ICOMP intersection determines t
OFF
, and hence D
OFF
. Since D
OFF
= V
IN
/V
OUT
by the
boost-topology equation, and since VIN is sinusoidal in wave-shape, and since ICOMP is proportional to the
inductor current, it follows that the control loop forces the inductor current to follow the input voltage wave-shape
to maintain boost regulation. Therefore, the average input current is also sinusoidal in wave-shape.
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