MAX1847EEE Maxim Integrated Products, MAX1847EEE Datasheet - Page 15

• Manufacturer Part Number: MAX1847EEE
• Description: Current Mode PWM Controllers
• Manufacturer: Maxim Integrated Products

Specifications of MAX1847EEE

• Case: ssop
• Dc: 05+
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

The ESR-induced ripple usually dominates this last
equation, so typically output capacitor selection is
based mostly upon the capacitor’s ESR, voltage rating,
and ripple current rating. Use the following formula to
determine the maximum ESR for a desired output ripple
voltage (V
Select a capacitor with ESR rating less than R
value of this capacitor is highly dependent on dielectric
type, package size, and voltage rating. In general, when
choosing a capacitor, it is recommended to use low-ESR
capacitor types such as ceramic, organic, or tantalum
capacitors. Ensure that the selected capacitor has suffi-
cient margin to safely handle the maximum RMS ripple
current.
For continuous inductor current the maximum RMS ripple
current in the output filter capacitor is:
The MAX1846/MAX1847 are externally loop-compen-
sated devices. This feature provides flexibility in
designs to accommodate a variety of applications.
Proper compensation of the control loop is important to
prevent excessive output ripple and poor efficiency
caused by instability. The goal of compensation is to
cancel unwanted poles and zeros in the DC-DC con-
verter’s transfer function created by the power-switch-
ing and filter elements. More precisely, the objective of
compensation is to ensure stability by ensuring that the
DC-DC converter’s phase shift is less than 180° by a
safe margin, at the frequency where the loop gain falls
below unity. One method for ensuring adequate phase
margin is to introduce corresponding zeros and poles
in the feedback network to approximate a single-pole
response with a -20dB/decade slope all the way to
unity-gain crossover.
The MAX1846/MAX1847 current-mode architecture
takes the double pole caused by the inductor and out-
put capacitor and shifts one of these poles to a much
higher frequency to make loop compensation easier.
To compensate these devices, we must know the cen-
ter frequencies of the right-half plane zero (z
the higher frequency pole (p
frequency with the following formula:
I
RMS
Choosing Compensation Components
RIPPLE-D
=
I
−
I
LOAD
R
D
ESR
______________________________________________________________________________________
MAX
):
= V
x
RIPPLE-D
Calculating Poles and Zeros
D
MAX
OUT2
). Calculate the z
/ I
−
LPP
D
MAX
2
High-Efficiency, Current-Mode,
RHP
ESR
) and
. The
RHP
Inverting PWM Controller
The calculations for p
applications where V
negative sense), the p
of the oscillator frequency and is generally at a higher
frequency than z
A pole is created by the output capacitor and the load
resistance. This pole must also be compensated and
its center frequency is given by the formula:
Finally, there is a zero introduced by the ESR of the out-
put capacitor. This zero is determined from the follow-
ing equation:
To ensure stability of the MAX1846/MAX1847, the gain
of the error amplifier must roll-off the total loop gain to 1
before Z
open-loop gain, A
where:
Select a unity-gain crossover frequency (f
is lower than z
Using f
(R
Determining the Compensation Component Values
Z
COMP
RHP
A
B is the feedback-divider attenuation factor =
G
400 µA/V
R
R
CS
O
CS
A
M
DC
R
is the error-amplifier output resistance = 3 MΩ
=
).
is the error-amplifier transconductance =
CROS
is the current sense amplifier gain = 3.3
is the selected current-sense resistor
COMP
RHP
−
=
p
⎡
⎢
⎢
⎣
Calculating the Required Pole Frequency
B x G
(
z
OUT1
1
ESR
or P
, calculate the compensation resistor
−
RHP
D
=
RHP
p
MAX
= 1 / (2π
DC
M
= 1 / (2π
OUT2
OUT2
A
and p
x
. Therefore:
:
DC
)
R
OUT
OUT2
2
OUT2
R
(
≥ 0.125
O
A
x V
2
occurs. First, calculate the DC
1
f
x P
CROS
R
π
CS
(
+
OUT2
x
does not exceed -48V (in a
2

x V

R
IN MIN
OUT
are very complex. For most
(
1
will not be lower than 1/8th
R
2
x R
(
C
−
OUT
LOAD
OUT
1
D
and higher than p

x R
CS
)
MAX
−
f
−
×
OSC

O
V
L

f
CROS
OUT
)
R
)
C
ESR
R
OUT
LOAD
)
x R
)
CROS
)
LOAD
), which
⎤
⎥
⎥
⎦
OUT1
15
.