NCL30001
www.onsemi.com
24
The gain of the low frequency current buffer is set by the
resistor at the I
AV G
pin, R
IAVG
. R
IAVG
sets the scaling factor
between the primary peak and primary average currents.
The gain of the current sense amplifier, A
CA
, is given by
Equation 5.
A
CA
+
RI
AVG
4k
(eq. 5)
The current sense signal is prone to leading edge spikes
during the switch turn on due to parasitic capacitance and
inductance. This spike may cause incorrect operation of the
PWM Comparator. The NCL30001 incorporates LEB
circuitry to block the first 200 ns (typical) of each current
pulse. This removes the leading edge spikes without filtering
the current signal waveform.
Oscillator
The oscillator controls the switching frequency, f, the jitter
frequency and the gain of the multiplier. The oscillator ramp
is generated by charging the timing capacitor on the CT Pin,
C
T
, with a 200 mA current source. This current source is
tightly controlled during manufacturing to achieve a
controlled and repeatable oscillator frequency. The current
source turns off and C
T
is immediately discharged with a
pull down transistor once the oscillator ramp reaches its peak
voltage, V
CT(peak)
, typically 4.0 V. The pull down transistor
turns off and the charging current source turns on once the
oscillator ramp reaches its valley voltage, V
CT(valley)
.
Figure 55 shows the resulting oscillator ramp and control
circuitry.
−
+
To PWM
comparator
To PWM skip
comparator
VDD
VDD
x 1.2
4.0 V / 0.1 V
+
−
+
Oscillator
Figure 55. Oscillator Ramp and Control Circuitry
Ramp Comp
R
RC
C
T
C
T
The relationship between the oscillator frequency in kHz
and timing capacitor in pF is given by Equation 6.
C
T
+
47000
f
(eq. 6)
A low frequency oscillator modulates the switching
frequency, reducing the controller EMI signature and
allowing the use of a smaller EMI filter. The frequency
modulation or jitter is typically ±6.8% of the oscillator
frequency.
