MAX8741EAI+T Maxim Integrated, MAX8741EAI+T Datasheet - Page 25

• 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

Table 5. Low-Voltage Troubleshooting Chart
improved by connecting V
such as the system 5V supply:
where t
V
This power is dissipated in the MOSFET body diode if
no external Schottky diode is used:
where I
Design Procedure and Input-Capacitor Value sections.
Under light loads, the PWM operates in discontinuous
mode, where the inductor current discharges to zero at
some point during the switching cycle. This makes the
inductor current’s AC component high compared to the
load current, which increases core losses and I
in the output filter capacitors. For best light-load efficien-
Sag or droop in V
under step-load change
Dropout voltage is too
high (V
V
Unstable—jitters between
different duty factors and
frequencies
Secondary output does
not support a load
Poor efficiency
Does not start under load
or quits before battery is
completely dead
FWD
IN
500kHz Multi-Output Power-Supply Controllers
decreases)
is the forward voltage of the diode.
OUT
SYMPTOM
RMS
D
Light-Load Efficiency Considerations
is the diode-conduction time (120ns typ) and
follows V
P(diode) = I
is the input ripple current as calculated in the
P(cap) = (I
OUT
______________________________________________________________________________________
IN
as
LOAD
Low V
differential, <1.5V
Low V
differential, <1V
Low V
differential, <0.5V
Low V
differential,
V
V
Low input voltage,
<5V
Low input voltage,
<4.5V
RMS
IN
OUT(MAIN)
L
✕
to an efficient 5V source,
< 1.3 x
CONDITION
)
V
2
IN
IN
IN
IN
FWD
with High Impedance in Shutdown
x R
- V
- V
- V
- V
OUT
OUT
OUT
OUT
ESR
✕
t
D
✕
f
2
R losses
Limited inductor-current slew rate
per cycle.
Maximum duty-cycle limits
exceeded.
Normal function of internal low-
dropout circuitry.
Not enough duty cycle left to
initiate forward-mode operation.
Small AC current in primary
cannot store energy for flyback
operation.
V
dropout and is not providing
good gate-drive levels.
V
V
L
L
L
linear regulator is going into
output is so low that it hits the
UVLO threshold.
ROOT CAUSE
cy, use MOSFETs with moderate gate-charge levels, and
use ferrite, MPP, or other low-loss core material.
The DC resistance (DCR) of the inductor can be used
to sense inductor current to improve the efficiency and
to reduce the cost by eliminating the sense resistor.
Figure 7 shows the sense circuit, where L is the induc-
tance, R
RC lowpass sense network. If the time constant of the
inductor is equal to that of the sense network, i.e.,:
then the voltage across C
where I
Determine the required sense-resistor value using the
equation given in the Current-Sense Resistor Value
section. Choose an inductor with DCR equal to or
greater than the sense resistor value. If the DCR is
greater than the sense-resistor value, use a divider to
L
L
is the inductor current.
is the inductor DCR, and R
Lossless-Inductor Current Sensing
Increase bulk output capacitance per
formula (see the Low-Voltage Operation
section). Reduce inductor value.
Reduce operation to 333kHz. Reduce
MOSFET on-resistance and coil DCR.
Increase the minimum input voltage or
ignore.
Reduce operation to 333kHz. Reduce
secondary impedances; use a Schottky
diode, if possible. Stack secondary
winding on the main output.
Use a small 20mA Schottky diode for
boost diode. Supply V
source.
Supply V
than V
R
V
L
S
L
=
IN
=
S
R
R C
, such as the system 5V supply.
L
becomes:
L
S S
from an external source other
×
I
L
SOLUTION
S
L
and C
from an external
S
form an
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