NCV890201 Datasheet, PDF (15/19 Pages) ON Semiconductor – 2.0 A, 2 MHz Automotive Buck Switching Regulator

NCV890201
with:
− I_OUT the average (dc) output current
− V_F the forward voltage of the diode
− I_DRMS the RMS current in the diode:
Ǹ ǒ Ǔ I_DRMS +
(1 * D)
I_OUT
2
)
DI_L ²
12
(eq. 6)
− R_D the dynamic resistance of the diode (extracted from
the V/I curve of the diode in its datasheet).
Then, knowing the thermal resistance of the package and
the amount of heatsinking on the PCB, the temperature rise
corresponding to this power dissipation can be estimated.
Input capacitor:
The input capacitor must sustain the RMS input ripple
current I_INac:
Ǹ I_INac
+
DI_L
2
D
3
VOUT
(eq. 7)
It can be designed in combination with an inductor to build
an input filter to filter out the ripple current in the source, in
order to reduce EMI conducted emissions.
For example, using a 4.7 mH input capacitor, it is easy to
calculate that an inductor of 200 nH will ensure that the
input filter has a cut−off frequency below 200 kHz (low
enough to attenuate the 2 MHz ripple).
Error Amplifier and Loop Transfer Function
The error amplifier is a transconductance type amplifier.
The output voltage of the error amplifier controls the peak
inductor current at which the power switch shuts off. The
Current Mode control method employed allows the use of a
simple, type II compensation to optimize the dynamic
response according to system requirements.
Figure 33 shows the error amplifier with the
compensation components and the voltage feedback divider.
RFB1
V_FB
V
RFB2
Vref
g_m * V
VCOMP
Cp
RO
RCOMP
CCOMP
Figure 33. Feedback Compensator Network Model
The transfer function from VOUT to VCOMP is the
product of the feedback voltage divider and the error
amplifier.
Gdivider(s)
+
RFB2
RFB1 ) RFB2
(eq. 8)
ǒ Ǔǒ Ǔ ^{Gerramp(s) + gm @ Ro @}
1
)
s
wz
1
)
s
wpl
1
)
s
wph
(eq. 9)
wz
+
RCOMP
1
@
CCOMP
wpl
+
Ro
@
1
CCOMP
(eq. 10)
(eq. 11)
wph
+
1
RCOMP
@
Cp
(eq. 12)
The output resistor Ro of the error amplifier is 1.4 MW and
gm is 1 mA/V. The capacitor Cp is for rejecting noise at high
frequency and is integrated inside the IC with a value of
18 pF.
The power stage transfer function (from Vcomp to output)
is shown below:
ǒ Ǔ ^Gps(s)
+
Rload
Ri
@
1
)
Rload@Tsw
L
@
1
[Mc
@
(1
*
D)
*
0.5]
@
1
)
s
wz
1
)
s
wp
@ Fh(s) (eq. 13)
wz
+
Resr
1
@
Cout
(eq. 14)
wp
+
1
Rload @
Cout
)
Mc @ (1 * D) * 0.5
L @ Cout @ Fsw
(eq. 15)
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