MAX742 Maxim, MAX742 Datasheet - Page 12

• Manufacturer Part Number: MAX742
• Description: Switch-Mode Regulator with +5V to 12V or 15V Dual Output
• Manufacturer: Maxim

Table 1. Trouble-Shooting Chart
Switch-Mode Regulator with
+5V to ±12V or ±15V Dual Output
12
Unstable Output.
Noise or jitter on
output ripple
waveform. Scope
may not trigger
correctly.
Noisy Output.
Switching is
steady, but large
inductive spikes
are seen at the
outputs.
Self-Destruction.
Transistors or IC
die on power-up.
Poor Efficiency.
Supply current is
high. Output will
not drive heavy
loads.
No Output. +VO
= 5V or less. -VO
= 0V.
No Switching.
±VO are correct,
but no waveform is
seen at LX+ or LX-.
SYMPTOM
______________________________________________________________________________________
Loop stability problem.
A. CC+ or CC- disconnected.
B. EMI: Move inductor away from IC or use
C. Grounding: Tie AGND directly to the filter
D. Bypass: Tie 10µF or larger between AGND
E. Current limiting: Reduce load currents.
F. Slope compensation: Inductor value not
A. Ground noise: Probe ground is picking up
B. Poor HF response: Add ceramic or
A. Input overvoltage: Never apply more
B. FB+ or FB- disconnected or shorted. This
C. CC+ or CC- shorted.
D. Output filter capacitor disconnected.
A. Inductor saturation: Peak currents
B. MOSFET on-resistance too high.
C. Switching losses: Diode is slow or has high
D. Inductor core losses: Hysteresis losses
E. Loop instability: See Unstable Output
A. Check connections. VREF should be +2V.
B. When input voltage is less than +4.2V,
Output is unloaded. Apply ±30mA or
greater load to observe waveform.
shielded inductors. Keep noise sources
away from CC- and CC+.
capacitor ground lead. Ensure that cur-
rent spikes from GND do not cause noise
at AGND or compensation capacitor or
reference bypass ground leads. Use wide
PC traces or a ground plane.
and VREF. Use 150µF to bypass the input
right at AV+. If there is high source resis-
tance, 1000µF or more may be required.
Ensure that inductors are not saturating.
matched to sense resistor.
switching EMI. Reduce probe ground lead
length (use probe tip shield) or put circuit
in shielded enclosure.
tantalum capacitors in parallel with output
filter capacitors.
than +12V.
causes runaway and output overvoltage.
exceed coil ratings.
forward voltage. Inductor has high DC resis-
tance. Excess capacitance at LX nodes.
cause self-heating in some core materials.
above.
undervoltage lockout is enabled.
CORRECTION
TRANSISTOR COUNT: 375
SUBSTRATE CONNECTED TO V+
___________________Chip Topography
100/200
12/15
VREF
AV+
AGND
SS
CC+ FB+ CSH+ CSL+
(2.03mm)
CC-
0.080"
FB-
CSL-
CSH-
GND
EXT+
PUMP
PDRV
EXT-
V+
(3.45mm)
0.135"