Applications information – Rainbow Electronics MAX1801 User Manual
Page 13
MAX1801
Digital Camera Step-Up Slave
DC-DC Controller
______________________________________________________________________________________
13
To configure the compensation network for a stable
control loop, set the crossover frequency at that of the
zero due to the output capacitor ESR. Use the following
procedure:
1) Determine the frequency of the right-half-plane
zero:
2) Find the DC loop gain:
A
VDC
= 2000 V
OUT
V
IN
3) Determine the frequency of the complex pole pair
due to the inductor and output capacitor:
f
O
= (V
OUT
/ V
IN
) / (2
π
(L C
OUT
)1/2)
4) Since response is 2nd order (-40dB per decade)
between the complex pole pair and the ESR zero,
determine the desired amplitude at the complex
pole pair to force the crossover frequency equal to
the ESR zero frequency. Thus:
A(P
O
) = (Z
O
/ P
O
)2 = L V
IN
2 / (C
OUT
ESR2 V
OUT
2)
5) Determine the desired compensation pole. Since
the response between the compensation pole and
the complex pole pair is 1st order (-20dB per
decade), the ratio of the frequencies is equal to the
ratio of the amplitudes at those frequencies. Thus:
(P
O
/ P
C
) = (A
DC
/ A(P
O
))
Solving this equation for C
C
:
C
C
= V
OUT
(C
OUT
)3/2 ESR2 / (20M
Ω V
IN
(L)
1/2
)
6) Determine that the compensation resistor, R
C
for
the compensation zero frequency, is equal to the
complex pole-pair frequency:
Z
C
= P
O
solving for R
C
:
R
C
= (V
IN
/ V
OUT
) ((L C
OUT
)
1/2
/ C
C
)
Applications Information
Using the MAX1801 with the MAX1800
Step-Up Master DC-DC Converter
The MAX1801 does not generate its own reference or
oscillator. Instead it uses the reference and the oscillator
from a master DC-DC converter such as the MAX1800
step-up master converter. The MAX1800 has circuitry to
generate a 1.25V reference and a 100kHz to 1MHz oscil-
lator signal. The MAX1800 operates from a 1.5V to 5.5V
input voltage, which makes it suitable for applications
with 2- or 3-cell alkaline, NiCd, or NiMH batteries, or 1-cell
lithium primary or lithium-ion (Li+) batteries. Apart from
the reference and the oscillator, the MAX1800 has a sin-
gle-internal-switch synchronous-rectified step-up DC-DC
converter, three auxiliary step-up DC-DC converter con-
trollers, and a linear regulator controller. For more details,
refer to the MAX1800 data sheet
Using the MAX1801 with the MAX1802
Step-Down Master DC-DC Converter
The MAX1801 does not generate its own reference or
oscillator. Instead, it uses the reference and the oscilla-
tor from a master DC-DC converter such as the
MAX1802 step-down master DC-DC converter. The
MAX1802 has circuitry to generate a 1.25V reference
and a 100kHz to 1MHz oscillator signal. The MAX1802
operates from a 2.7V to 11V input voltage, making it
suitable for 4-series alkaline, NiCd, or NiMH cells, or 2-
series lithium primary or (Li+) batteries. The MAX1802
has a synchronous-rectified step-down DC-DC convert-
er controller, an internal-switch synchronous-rectified
step-down DC-DC converter, and three auxiliary step-
up DC-DC converter controllers. For more details, refer
to the MAX1802 data sheet.
Z
(1 - D) R
2 L
RHP
2
LOAD
=
π
FREQUENCY
A
VDC
GAIN
(dB)
PHASE
180
°
90
°
0
°
O dB
PHASE
GAIN
Z
C
=P
O
PC
Z
0
Z
rRHP
PHASE
MARGIN
GAIN
MARGIN
Figure 5. MAX1801 Continuous-Current, Voltage-Mode,
Step-Up Converter Bode Plot