MAX8513 MAXIM [Maxim Integrated Products], MAX8513 Datasheet - Page 22

• Manufacturer Part Number: MAX8513
• Description: Wide-Input, High-Frequency, Triple-Output Supplies with Voltage Monitor and Power-On Reset
• Manufacturer: MAXIM [Maxim Integrated Products]

Wide-Input, High-Frequency, Triple-Output Supplies
with Voltage Monitor and Power-On Reset
To set the current limit and the current-limit foldback
thresholds, first select the foldback current-limit ratio
(P
divided by the current limit when V
inal regulated voltage (I
P
R17 and R18 (in the Typical Applications Circuits), use
the following equations:
R
high operating temperature. R
series resistance of the inductor or a discrete current-
sense resistor value. I
at maximum load, which equals:
If R18 results in a negative resistance, then decrease
R
lower DC resistance or a lower value discrete current-
sense resistor.
For constant current-limit operation, connect ILIM to VL
for a default current-limit threshold of 170mV (typ). The
sensing resistor value must then be chosen so that:
the minimum value of the default threshold.
Alternately, the constant current-limit threshold can also
be set by using only R18, in which case R18 is calculat-
ed as follows:
When using the DC resistance of the inductor as a cur-
rent-sense resistor, an RC filter is needed (R19 and
22
FB
CS
CS_MAX
FB
R
. This can be done by choosing an inductor with a
). This ratio is the foldback current limit (I
______________________________________________________________________________________
is typically set to 0.5. To calculate the values of
18
=
is the maximum sensing resistance at the
R
V
(
18
OUT
7 5
.
R
CS_MAX
=
1
I
×
R
OUT MAX
-
7 5
17
R
(
.
P
7 5
CS MAX
FB
.
=
1
_
×
LIMIT
_
4 7
×
=
LIMIT
(
.
× I
P
R
R
I
LIMIT
FB
CS MAX
µ
Constant Current Limit
CS MAX
LIMIT
I
A
LIMIT
is the peak inductor current
×
).
×
_
_
×



×
I
@0
LIMIT
1
< 151mV
V
(
+
1
CS
OUT
V
2
-
LIR
×
OUT1
×
P
×
FB
I
can either be the
1
LIMIT
4 7
I



(
LIMIT
)
1
.
)
-
equals its nom-
µ
P
A
FB
×
(
)
1
)
LIMIT@0V
-
×
P
R
FB
17
)
)
)
C14 of the Typical Applications Circuits). Pick the value
of the filter capacitor, C14, from 0.22µF to 1µF (ceramic
X7R). Then calculate the value of R19 as follows:
R
tance. Additionally, R20 (in the Typical Applications
Circuits) is added in series with the CSN input to cancel
the drop due to input bias current into CSP that devel-
ops across R19. R20 should be set equal to R19.
The MAX8513/MAX8514 use a voltage-mode control
scheme that regulates the output voltage by comparing
the error-amplifier output (COMP) with a fixed internal
ramp to produce the required duty cycle. The output
lowpass LC filter creates a double pole at the resonant
frequency, which has a gain drop of -40dB/decade and
a phase shift of approximately -180°/decade. The error
amplifier must compensate for this gain drop and
phase shift to achieve a stable high-bandwidth closed-
loop system.
The basic regulator loop consists of a power modulator,
an output feedback divider, and an error amplifier. The
power modulator has a DC gain set by V
(V
set by the output inductance (L), the output capaci-
tance (C
and its equivalent series resistance (R
the peak of the saw-toothed waveform at the input of
the PWM comparator (see the Functional Diagrams in
Figures 1 and 2). Below are equations that define the
power modulator:
where L is L1A and C
tions Circuits.
L_DC
RAMP
is the nominal value of the inductor’s DC resis-
= 1V
OUT
f
ZESR
) (C4 in the Typical Applications Circuits),
P-P
R
G
f
PMOD
19
MOD DC
), with a double pole and a single zero
=
=
(
(
2
2π
=
OUT
×
)
2π
R
×
=
L DC
is C4 in the Typical Applica-
Compensation Design
C
_
L A
V
L
OUT
1
RAMP
V
×
IN
1
1
×
C
×
OUT
C
14
R
ESR
)
ESR
). V
IN
/ V
RAMP
RAMP
is