MIC25400 Datasheet, PDF (19/27 Pages) Micrel Semiconductor – 2A Dual Output PWM Synchronous Buck Regulator IC

Micrel, Inc.
The G_DC is the DC gain of the error amplifier. It is
internally set to 2500 (68dB).
As illustrated in Figure 12, there are two compensating
zeros. ωz1 is internally set with R3 and C3. The zero
frequency is fixed at a nominal 16kHz in the MIC25400.
The second zero, ωz2, is set by the external capacitor,
C2.
For the MIC25400:
R3 = 100k
C3 = 100pf
fz1 =
1
= 16kHz
2 × π × R3 × C3
fz2 =
1
2 × π × 21⋅ 10³ × C2
The two compensating pole frequencies are shown
below.
fp1 = 250Hz
fp2 =
1
2 × π × 12 ⋅ 10³ × C2
fp2 and fz2 both depend on the value of C2 and are
proportionally spaced in frequency with the zero at a
lower frequency than the pole. This provides gain and
phase boost in the control loop.
MIC25400
Voltage Divider Feedforward Capacitor
The capacitor across the upper voltage divider resistor
boosts the gain and phase of the control loop by short
circuiting the high-side resistor at higher frequencies.
The capacitor and upper resistor form a zero at a lower
frequency. The capacitor and parallel combination of
upper and lower resistors form a pole at a higher
frequency. This phase boost circuit is most effective at
higher output voltages, where there is a larger
attenuation from the voltage divider resistors. The
general form of the feedforward circuit is shown below.
⎜⎛1 + ^s ⎟⎞
H(s) = ^R2 × ^{⎝ ωz3 âŽ} 
R1 + R2
⎜⎜⎝⎛^{1 +}
s
ωp3
⎟⎟⎠⎞
Where:
fz3 =
1
2 × π × R1× C1
fp3 =
1
2 × π × C1× ⎜⎛ ^{R1× R2} ⎟⎞
⎝ ^{R1 + R2} ⎠
The total open loop transfer function is:
T(s) = Gea(s) × Gmod × Gfilter(s) × H(s)
The following tables list the recommended values of
compensation and filter components for different output
voltages. The output capacitors are ceramic.
MIC25400
VIN VOUT R1 R2 C2 C1 Lo Co
12V 1.0V 1k 2.32k 47pF 1.5nF 4.7µH 22µF
12V 1.2V 1k 1.4k 47pF 1.5nF 4.7µH 22µF
12V 1.4V 1k 1k 47pF 1.5nF 4.7µH 22µF
12V 1.8V 1k 634 47pF 1.5nF 4.7µH 22µF
12V 2.5V 1k 383 47pF 3.3nF 4.7µH 22µF
12V 3.3V 1k 274 68pF 3.3nF 4.7µH 22µF
12V 5.0V 1k 162 68pF 3.3nF 4.7µH 22µF
January 2011
19
M9999-020111-C