MAX8741EAI+T Maxim Integrated, MAX8741EAI+T Datasheet - Page 22

• Manufacturer Part Number: MAX8741EAI+T
• Description: Current & Power Monitors & Regulators 500kHz Multi-Out Pwr Supply Controller
• Manufacturer: Maxim Integrated

Specifications of MAX8741EAI+T

• Rohs: yes
• Product: Power Monitors
• Supply Voltage - Max: 30 V
• Supply Voltage - Min: 4.2 V
• Input Voltage Range: 4.2 V to 30 V

500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
Bypass the V+ input with a 4.7µF tantalum capacitor
paralleled with a 0.1µF ceramic capacitor, close to the
IC. A 10Ω series resistor to V
Bypass the V
paralleled with a 0.1µF ceramic capacitor, close to the
device.
The output-filter capacitor values are generally deter-
mined by the ESR and voltage-rating requirements,
rather than actual capacitance requirements for loop sta-
bility. In other words, the low-ESR electrolytic capacitor
that meets the ESR requirement usually has more output
capacitance than is required for AC stability. Use only
specialized low-ESR capacitors intended for switching-
regulator applications, such as AVX TPS, Sanyo
POSCAP, or Kemet T510. To ensure stability, the capaci-
tor must meet both minimum capacitance and maximum
ESR values as given in the following equations:
These equations are worst case, with 45° of phase mar-
gin to ensure jitter-free, fixed-frequency operation and
provide a nicely damped output response for zero to
full-load step changes. Some cost-conscious designers
may wish to bend these rules with less-expensive
capacitors, particularly if the load lacks large step
changes. This practice is tolerable if some bench test-
ing over temperature is done to verify acceptable noise
and transient response.
No well-defined boundary exists between stable and
unstable operation. As phase margin is reduced, the
first symptom is a bit of timing jitter, which shows up as
blurred edges in the switching waveforms where the
scope does not quite sync up. Technically speaking,
this jitter (usually harmless) is unstable operation, since
the duty factor varies slightly. As capacitors with higher
ESRs are used, the jitter becomes more pronounced, and
the load-transient output-voltage waveform starts looking
ragged at the edges. Eventually, the load-transient wave-
form has enough ringing on it that the peak noise levels
exceed the allowable output-voltage tolerance. Note that
even with zero phase margin and gross instability pre-
sent, the output-voltage noise never gets much worse
than I
22
______________________________________________________________________________________
PEAK
C
OUT
✕
R
L
ESR
>
output with a 4.7µF tantalum capacitor
V
R
Output-Filter Capacitor Value
REF
ESR
(under constant loads).
V
OUT
(
1
<
+
R
V
×
SENSE
OUT
R
SENSE
IN
V
/
REF
is also recommended.
V
×
IN MIN
V
(
×
OUT
f
Bypassing V+
Bypassing V
)
)
L
The output-voltage ripple is usually dominated by the
filter capacitor’s ESR, and can be approximated as
I
full equation for ripple in continuous-conduction mode
is V
C
discontinuous, with high peaks and widely spaced
pulses, so the noise can actually be higher at light load
(compared to full load). In idle mode, calculate the out-
put ripple as follows:
Buck-plus-flyback applications, sometimes called “cou-
pled-inductor” topologies, need a transformer to gener-
ate multiple output voltages. Performing the basic
electrical design is a simple task of calculating turns
ratios and adding the power delivered to the secondary
to calculate the current-sense resistor and primary
inductance. However, extremes of low input-output dif-
ferentials, widely different output loading levels, and
high turns ratios can complicate the design due to par-
asitic transformer parameters such as interwinding
capacitance, secondary resistance, and leakage induc-
tance. For examples of what is possible with real-
world transformers, see the Maximum V
Current vs. Input Voltage graph in the Typical Operating
Characteristics.
Power from the main and secondary outputs is com-
bined to get an equivalent current referred to the main
output voltage (see the Inductor Value section for para-
meter definitions). Set the current-sense resistor value
at 80mV / I
P
I
referred to V
RIPPLE
TOTAL
TOTAL
OUT
NOISE(P-P)
L
Turns Ratio N
)]. In idle mode, the inductor current becomes
PRIMARY
= P
✕
= the sum of the output power from all outputs
R
V
0 0003
TOTAL
TOTAL
NOISE P P
ESR
.
OUT
=
. There is also a capacitive term, so the
V
= I
.
IN MAX
(
/ V
× ×
V
(
-
OUT
L
=
RIPPLE
OUT
R
(for Auxiliary Outputs Only)
)
V
SENSE
=
)
OUT MIN
(
[ /
V
1
0 025
× ×
IN MAX
= the equivalent output current
.
V
f I
(
(
R
OUT
SENSE
2
✕
V
TOTAL
SEC
×
)
×
Transformer Design
)
[R
+
C
-
+
R
V
V
OUT
ESR
1
ESR
+
RECT
OUT
/(
V
×
V
FWD
IN
LIR
)
+
+ 1/(2
+
-
V
V
SENSE
OUT
DD
)]
✕
π
Output
✕
f
✕