MAX1945REUI Maxim Integrated Products, MAX1945REUI Datasheet - Page 14

• Manufacturer Part Number: MAX1945REUI
• Description: DC/DC Switching Regulators 1MHz 1% 6A Step-Down PWM
• Manufacturer: Maxim Integrated Products

Specifications of MAX1945REUI

• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The double pole formed by the inductor and the output
capacitor of most voltage-mode controllers introduces
a large phase shift, which requires an elaborate com-
pensation network to stabilize the control loop. The
MAX1945R/MAX1945S controllers utilize a current-
mode control scheme that regulates the output voltage
by forcing the required current through the external
inductor, eliminating the double pole caused by the
inductor and output capacitor, and greatly simplifying
the compensation network. A simple Type 1 compensa-
tion with a single compensation resistor (R
pensation capacitor (C
bandwidth loop (Figure 1).
An internal transconductance error amplifier compen-
sates the control loop. Connect a series resistor and
capacitor between COMP (the output of the error
amplifier) and GND, to form a pole-zero pair. The external
inductor, internal current-sense circuitry, output capaci-
tor, and external compensation circuit determine the
loop-system stability. Choose the inductor and output
capacitor based on performance, size, and cost.
Additionally, select the compensation resistor and capac-
itor to optimize control-loop stability. The component val-
ues shown in the typical application circuit yield stable
operation over a broad range of input-to-output voltages.
Compensating the voltage feedback loop depends on
the type of output capacitors used. Common capaci-
tors for output filtering: ceramic capacitors, polymer
capacitors such as POSCAPs and SPCAPs, and elec-
trolytic capacitors. Use either ceramic or polymer
capacitors. Use polymer capacitors as the output
capacitor when selecting 500kHz operation. At 500kHz
switching, the voltage feedback loop is slower (about
50kHz to 60kHz) when compared to 1MHz switching.
Therefore, a polymer capacitor’s high capacitance for a
given footprint improves the output response during a
step load change. Because of its relative low ESR fre-
quencies (about 20kHz to 80kHz), use Type 2 compen-
sation. The additional high-frequency pole introduced
in Type 2 compensation offsets the ESR zero intro-
duced by the polymer capacitors to provide continuous
attenuation above the ESR zero frequencies of the poly-
mer capacitors. However, the presence of the parasitic
capacitance at COMP and the high output impedance
of the error amplifier already provide the required atten-
uation above the ESR frequencies. The following steps
outline the design process of compensating the
MAX1945 with polymer output capacitors with the com-
ponents in the application circuits Figures 1 and 2.
1MHz, 1% Accurate, 6A Internal Switch
Step-Down Regulators
14
______________________________________________________________________________________
C
) creates a stable and high
Compensation Design
C
) and com-
Regulator DC Gain:
G
Load Impedance Pole Frequency:
fp
Load Impedance Zero Frequency:
fz
where R
The feedback divider has a gain of G
where V
er has a DC gain, G
capacitor, C
amplifier, R
R
pole frequency as:
Determine the compensation zero frequency as:
For best stability and response performance, set the
closed-loop unity-gain frequency much higher than the
load-impedance pole frequency. The closed-loop unity-
gain crossover frequency must be less than one-fifth of
the switching frequency. Set the crossover frequency to
10% to 15% of the switching frequency. The loop-gain
equation at unity-gain frequency, f
where G
transconductance of the voltage-error amplifier.
Calculate R
C
DC
ESR
LOAD
R
set a compensation zero. Calculate the dominant
C
= ∆V
= 1/(2
G
= (V
= 1/(2
OUT
EA
FB
EA
OUT
✕
OUT
= 0.8V. The transconductance error amplifi-
OEA
C
✕
= V
C
G
= gm
as:
/∆V
, and the output resistance of the error
✕
π
DC
fp = 1/(2π
✕
(20MΩ), set the dominant pole. C
fz
π
✕
OUT
COMP
EA
f
✕
C
C
✕
EA
)/(gm
OUT
C
EA(DC)
/I
(f
= 1/(2π
OUT(MAX)
OUT
PLOAD
✕
= gmc
✕
EA
R
✕
R
✕
, of 70dB. The compensation
C
ESR
/f
✕
(R
C
, and gm
✕
C
C
V
OUT
C
)
✕
, and gmc = 18.2S.
FB
)
✕
C
✕
C
R
, is given by:
✕
R
(V
✕
OUT
+ R
OEA
✕
R
FB
G
C
/V
ESR
EA
)
DC
)
FB
OUT
))
= 50µS, the
✕
= V
f
) = 1
PLOAD
FB
/V
C
OUT
)
and
,