MAX1965TEEP Maxim Integrated, MAX1965TEEP Datasheet - Page 20

• Manufacturer Part Number: MAX1965TEEP
• Description: Current & Power Monitors & Regulators
• Manufacturer: Maxim Integrated

where V
er’s gain (4.9 typ), A
ductance error amplifier (2000 typ) set by its DC output
resistance, and V
set by the feedback resistive-divider (internal or exter-
nal). Since the output voltage is a function of the load
current and load resistance, the total DC loop gain
(A
The first compensation capacitor (C
dominant pole. Due to the current-mode control
scheme, the output capacitor also creates a pole in the
system which is a function of the load resistance. As
the load resistance increases, the frequency of the out-
put capacitor’s pole decreases. However, the DC loop
gain increases with larger load resistance, so the unity-
gain bandwidth remains fixed. Additionally, the com-
pensation resistor and the output capacitor’s ESR both
generate zeros which must be canceled out by corre-
sponding poles. Therefore, in order to achieve stable
operation, use the following procedure to properly com-
pensate the system:
1) The crossover frequency (the frequency at which
2) Determine the series compensation capacitor
3) Before crossover occurs, the output capacitor and
Tracking/Sequencing Triple/Quintuple
Power-Supply Controllers
20
V(DC)
A
unity gain occurs) must be less than 1/5th the
switching frequency:
(C
quency:
where the error amplifier’s transconductance (g
is 100µS (see Electrical Characteristics) and A
is the total DC loop gain defined above.
the load resistor generate a second pole:
______________________________________________________________________________________
V DC
COMP1
(
) is approximately:
REF
I
PEAK
)
≈
≈
= 1.24V, A
I
I
V
) required to set the desired crossover fre-
LOAD
PEAK
OUT NOMINAL DS ON
=
C
400
COMP
V
(
OUT(NOMINAL)
OUT NOMINAL DS ON
≈
×
VEA
V
V
(
1
OUT NOMINAL DS ON
REF LOAD
VCS
V
≥
f
is the DC gain of the transcon-
C
OUT REF VEA
2
R
(
≤
1
V
π
is the current-sense amplifi-
)
R
REF LOAD VEA
f
SW
V



5
g A
)
m V DC
R
(
2000
R
is the output voltage
A
)
)
COMP1
(
R
(
f
C
A
)
)
A
(



VCS
) creates the
)
A
VCS
V(DC)
m
)
4) The series compensation resistor and capacitor
5) For most applications using electrolytic capacitors,
6) Finally, if the output capacitor’s ESR zero occurs
For example, the MAX1964 Standard Application
Circuit shown in Figure 1 requires a 5V output that sup-
ports up to 2A. Using the above compensation guide-
lines, we can determine the proper component values:
•
•
•
provide a zero which can be used to cancel the
second pole in order to ensure stability:
the output capacitor’s ESR forms a second zero
that occurs before crossover. Applications using
low-ESR capacitors (e.g., polymer, OS-CON) may
have ESR zeros that occur after crossover.
Therefore, verify the frequency of the output capaci-
tor’s ESR zero:
before crossover, add the parallel compensation
capacitor (C
this second zero:
First, select the crossover frequency to be 1/5th the
200kHz switching frequency.
Next, determine the total DC loop gain (A
you can calculate the series compensation capaci-
tance (C
the International Rectifier IRF7101 with an R
of 100mΩ, the DC loop gain approximately equals
2480 and C
Select the closest standard capacitor value of
470pF.
Determine the location of the output pole
(f
1000µF electrolytic capacitor, the output pole
occurs at 64Hz.
POLE(OUT)
f
C
POLE OUT
COMP
(
COMP1
R
2
COMP
f
ZERO ESR
≈
≈
). With a 5V output supplying 2A and a
)
COMP1
COMP2
=
(
(
2
f
ZERO ESR
2
). Since the applications circuit uses
π
(
C
π
≥
R
C
COMP POLE OUT
COMP COMP ZERO ESR
2
OUT LOAD
) to form a third pole to cancel
π
(
must be approximately 490pF.
)
C
≈
1
COMP POLE OUT
R
1
2π
C
)
f
-
C
C
f
POLE OUT
COMP
OUT ESR
1
1
f
1
(
=
1
R
(
f
2
1
I
π
LOAD MAX
(
)
C
)
OUT OUT
)
(
)
(
V
)
V(DC)
-
)
1
DS(ON)
)
) so