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MAX8742EAI+T

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34
MAX8741/MAX8742
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
VRI
SLL
=×
L
R
RC
L
SS
=
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
MAX8741/MAX8742
500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
26 ______________________________________________________________________________________
scale down the voltage. Use the maximum inductance
and minimum DCR to get the maximum possible induc-
tor time constant. Select R
S
and C
S
so that the maxi-
mum sense-network time constant is equal to or greater
than the maximum inductor time constant.
Reduced Output-Capacitance Application
In applications where higher output ripple is accept-
able, lower output capacitance or higher ESR output
capacitors can be used. In such cases, cycle-by-cycle
stability is maintained by adding feed-forward compen-
sation to offset for the increased output ESR. Figure 8
shows the addition of the feed-forward compensation
circuit. C
FB
provides noise filtering, R
FF
is the feed-for-
ward resistor, and C
LX
provides DC blocking. Use
100pF for C
FB
and C
LX
. Select R
FF
according to the
equation below:
Set the value for R
FF
close to the calculation. Do not
make R
FF
too small as that introduces too much feed-
forward, possibly causing an overvoltage to be seen at
the feedback pin, and changing the mode of operation
to a voltage mode.
PC Board Layout Considerations
Good PC board layout is required in order to achieve
specified noise, efficiency, and stability performance.
The PC board layout artist must be given explicit
instructions, preferably a pencil sketch showing the
placement of power-switching components and high-
current routing. A ground plane is essential for optimum
performance. In most applications, the circuit is located
on a multilayer board, and full use of the four or more
copper layers is recommended. Use the top layer for
high-current connections, the bottom layer for quiet
connections (REF, SS, GND), and the inner layers for
an uninterrupted ground plane. Use the following step-
by-step guide:
1) Place the high-power components (Figure 1, C1, C3,
C4, Q1, Q2, L1, and R1) first, with their grounds
adjacent:
• Priority 1: Minimize current-sense resistor trace
lengths and ensure accurate current sensing with
Kelvin connections (Figure 9).
• Priority 2: Minimize ground trace lengths in the
high-current paths (discussed below).
• Priority 3: Minimize other trace lengths in the
high-current paths.
a) Use >5mm-wide traces
b) CIN to high-side MOSFET drain: 10mm max
length
c) Rectifier diode cathode to low-side MOSFET:
5mm max length
R
RLf
ESR
FF
≤
×××43
L
DL_
DH_
LX_
MAX8741
MAX8742
CSH_
CSL_
INDUCTOR
R
L
V
OUT
V
IN
C
IN
C
OUT
C
S
R
S
Figure 7. Lossless Inductor Current Sensing
R3
R4
FB_
L
C
IN
DL_
DH_
LX_
MAX8741
MAX8742
CSH_
CSL_
V
IN
C
LX
R
FF
C
FB
R
SENSE
V
OUT
C
OUT
Figure 8. Adding Feed-Forward Compensation
MAX8741/MAX8742
500kHz Multi-Output Power-Supply Controllers
with High Impedance in Shutdown
______________________________________________________________________________________ 27
d) LX node (MOSFETs, rectifier cathode, induc
tor): 15mm max length
Ideally, surface-mount power components are butted up
to one another with their ground terminals almost touch-
ing. These high-current grounds are then connected to
each other with a wide filled zone of top-layer copper so
they do not go through vias. The resulting top layer “sub-
ground-plane” is connected to the normal inner-layer
ground plane at the output ground terminals, which
ensures that the IC’s analog ground is sensing at the sup-
ply’s output terminals without interference from IR drops
and ground noise. Other high-current paths should also
be minimized, but focusing primarily on short ground and
current-sense connections eliminates about 90% of all PC
board layout problems.
2) Place the IC and signal components. Keep the main
switching nodes (LX nodes) away from sensitive
analog components (current-sense traces and REF
capacitor). Place the IC and analog components on
the opposite side of the board from the power-
switching node. Important: The IC must be no more
than 10mm from the current-sense resistors. Keep
the gate-drive traces (DH_, DL_, and BST_) shorter
than 20mm and route them away from CSH_, CSL_,
and REF.
3) Use a single-point star ground where the input
ground trace, power ground (subground plane), and
normal ground plane meet at the supply’s output
ground terminal. Connect both IC ground pins and all
IC bypass capacitors to the normal ground plane.
MAX8741/MAX8742
SENSE RESISTOR
HIGH-CURRENT PATH
Figure 9. Kelvin Connections for the Current-Sense Resistors
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24P25-P27P28-P30P31-P33P34-P34

MAX8742EAI+T

Mfr. #:
Buy MAX8742EAI+T
Manufacturer:
Maxim Integrated
Description:
Current & Power Monitors & Regulators 500kHz Multi-Out Pwr Supply Controlle
Lifecycle:
New from this manufacturer.
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