Rainbow Electronics MAX8742 - Manual (Page 25)

MAX8741/
M
AX8742
500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
______________________________________________________________________________________ 25
improved by connecting V L to an efficient 5V source,
such as the system 5V supply:
P(diode) = I LOAD ✕ V FWD ✕ t D ✕ f
where t D is the diode-conduction time (120ns typ) and
V FWD is the forward voltage of the diode.
This power is dissipated in the MOSFET body diode if
no external Schottky diode is used:
P(cap) = (I RMS ) 2 x R ESR
where I RMS is the input ripple current as calculated in the
Design Procedure and Input-Capacitor Value sections.
Light-Load Efficiency Considerations
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
2 R losses
in the output filter capacitors. For best light-load efficien-
cy, use MOSFETs with moderate gate-charge levels, and
use ferrite, MPP, or other low-loss core material.
Lossless-Inductor Current Sensing
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
L is the inductor DCR, and R S and C S form an
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 S becomes:
where I L is the inductor current.
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
V R I
S L L
= ×
L
R
R C
L
S S
=
SYMPTOM CONDITION ROOT CAUSE SOLUTION
Sag or droop in V OUT
under step-load change
Low V IN - V OUT
differential, <1.5V
Limited inductor-current slew rate
per cycle.
Increase bulk output capacitance per
formula (see the Low-Voltage Operation
section). Reduce inductor value.
Dropout voltage is too
high (V
OUT follows V IN as
V
IN
decreases)
Low V IN - V OUT
differential, <1V
Maximum duty-cycle limits
exceeded. Reduce operation to 333kHz. Reduce
MOSFET on-resistance and coil DCR.
Unstable—jitters between
different duty factors and
frequencies
Low V IN - V OUT
differential, <0.5V
Normal function of internal low-
dropout circuitry. Increase the minimum input voltage or
ignore.
Secondary output does
not support a load
Low V IN - V OUT
differential,
V
IN < 1.3 x
V OUT(MAIN)
Not enough duty cycle left to
initiate forward-mode operation.
Small AC current in primary
cannot store energy for flyback
operation.
Reduce operation to 333kHz. Reduce
secondary impedances; use a Schottky
diode, if possible. Stack secondary
winding on the main output.
Poor efficiency
Low input voltage,
<5V
V L linear regulator is going into
dropout and is not providing
good gate-drive levels.
Use a small 20mA Schottky diode for
boost diode. Supply V
L from an external
source.
Does not start under load
or quits before battery is
completely dead
Low input voltage,
<4.5V V L output is so low that it hits the
V L UVLO threshold.
Supply V L from an external source other
than V IN , such as the system 5V supply.
Table 5. Low-Voltage Troubleshooting Chart