LV5980MX Datasheet, PDF (11/16 Pages) Sanyo Semicon Device – Low power consumption and high efficiency Step-down Switching Regulator

LV5980MX
Calculation of external phase compensation constant
Generally, to stabilize switching regulator, the frequency where closed loop gain is 1 (zero-cross frequency fZC) should
be 1/10 of the switching frequency (or 1/5). Since the switching frequency of this IC is 370kHz, the zero-cross frequency
should be 37kHz. Based on the above condition, we obtain the following formula (13).
VREF
1
RL
VOUT ^{• GMER • RC +} SCC ^{• GCS •} 1 + SCO • RL ^{= 1}
(13)
As for zero-cross frequency, since the impedance element of phase compensation is RC >>1/SCC, the following equation
(14) is obtained.
VREF
RL
VOUT ^{• GMER • RC • GCS •} 1 + 2π • fZC • CO • RL ^{= 1}
(14)
Phase compensation external resistance can be obtained with the following formula (15), the variation of the formula (14).
Since 2π • fZC • CO • RL >> 1 in the equation (15), we know that the external resistance is independent of load resistance.
VOUT ₁
1 ^{1 + 2π • fZC • CO • RL}
RC = _VREF • _GMER • _GCS •
RL
(15)
When output is 5V and load resistance is 5Ω (1A load), the resistances of phase compensation are as follows.
GCS = 2.7A/V, GMER = 220μA/V, fZC = 37kHz
RC
=
5
1.235
×
220
1
×
10^-6
×
1
2.7
×
1
+
2
×
3.14
×
(37
×
10³)
5
×
(30
×
10^-6)
×
5
=
48.898…×
10³
= 48.90 [kΩ]
If frequency of zero point fz and pole fp1 are in the same position, they cancel out each other. Therefore, only the pole
frequency remains for frequency characteristics of the closed loop gain.
In other words, gain decreases at -20dB/dec and phase only rotates by 90º and this allows characteristics where oscillation
never occurs.
fp1 = fz
1
1
2π • CO • RL ^•2π • CO • RC
CC
=
RL • CO
RC
•
5
× (30 × 10^-6)
48.9 × 10³
=
3.067…×
10^-9
= 3.07 [nF]
The above shows external compensation constant obtained through ideal equations. In reality, we need to define phase
constant through testing to verify constant IC operation at all temperature range, load range and input voltage range. In the
evaluation board for delivery, phase compensation constants are defined based on the above constants. The zero-cross
frequency required in the actual system board, in other word, transient response is adjusted by external compensation
resistance. Also, if the influence of noise is significant, use of external phase compensation capacitor with higher value is
recommended.
No.A1980-11/16