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MAX15021ATI Datasheet, PDF (17/24 Pages) Maxim Integrated Products – Dual, 4A/2A, 4MHz, Step-Down DC-DC Regulator | |||
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MAX15021
Dual, 4A/2A, 4MHz, Step-Down DC-DC
Regulator with Tracking/Sequencing Capability
Type II: Compensation when fCO > fZERO,ESR
When the fCO is greater than fESR, a Type II compensa-
tion network provides the necessary closed-loop com-
pensated response. The Type II compensation network
provides a midband compensating zero and a high-fre-
quency pole (see Figures 5a and 5b).
RFCF provides the midband zero fMID,ZERO, and
RFCCF provides the high-frequency pole, fHIGH,POLE.
Use the following procedure to calculate the compen-
sation network components.
Calculate the fESR and LC double pole, fLC:
fESR
=
2Ï
Ã
1
ESR Ã
COUT
fLC â
1
2Ï Ã L Ã COUT
where COUT is the regulator output capacitor and ESR
is the series resistance of COUT. See the Output-
Capacitor Selection section for more information on cal-
culating COUT and ESR.
Set the compensatorâs leading zero, fZ1, at or below the
filterâs resonant double-pole frequency from:
fZ1 ⤠fLC
Set the compensatorâs high-frequency pole, fP1, at or
below one-half the switching frequency, fSW:
fP1
â¤
fSW
2
To maximize the compensatorâs phase lead, set the
desired crossover frequency, fCO, equal to the geomet-
ric mean of the compensatorâs leading zero, fZ1, and
high-frequency pole, fP1, as follows:
fCO = fZ1 Ã fP1
Select the feedback resistor, RF, in the range of 3.3kΩ
to 30kΩ.
Calculate the gain of the modulator (GainMOD)âcom-
prised of the regulatorâs pulse-width modulator, LC filter,
feedback divider, and associated circuitryâat the desired
crossover frequency, fCO, using the following equation:
( ) GainMOD = 4(V/V)Ã
ESR [mΩ]
2Ï Ã fCO[kHz] à L[μH]
à VFB [V]
VOUT_ [V]
where VFB is the 0.6V (typ) FB_ input-voltage set-point,
L is the value of the regulator inductor, ESR is the
VOUT_
R1
CCF
RF
CF
FB_
R2
VREF
Figure 5a. Type II Compensation Network
GAIN
(dB)
COMP_
1ST ASYMPTOTE
(ÏR1CF)-1
2ND ASYMPTOTE
(RFR1)-1
3RD ASYMPTOTE
(ÏRFCCF)-1
1ST POLE
(AT ORIGIN)
1ST ZERO
(RFCF)-1
2ND POLE
(RFCCF)-1
Ï (rad/sec)
Figure 5b. Type II Compensation Network Response
series resistance of the output capacitor, and VOUT_ is
the desired output voltage.
The gain of the error amplifier (GainE/A) in the midband
frequencies is:
GainE/A
=
RF
R1
[kΩ]
[kΩ]
The total loop gain is the product of the modulator gain
and the error amplifier gain at fCO and should be set
equal to 1 as follows:
GainMOD x GainE/A = 1
So:
20
Ã
log10
â¡
â¢
â£
RF
R1
â¤
â¥
â¦
+
20
Ã
log10
â¡
â¢
â£â¢
2
4 ÃESR
ÏÃfCO ÃL
x VFB
x VOUT_
â¤
â¥
â¦â¥
= 0dB
RF Ã 4 Ã ESR x VFB = 1
R1 2Ï Ã fCO Ã L x VOUT_
Maxim Integrated
17
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