Hig h -Effic ie n c y, P WM, S t e p -Do w n
DC-DC Co n t ro lle rs in 1 6 -P in QS OP
2–MAX165
Bo o s t -S u p p ly Dio d e D2
_____________Lo w -Vo lt a g e Op e ra t io n
A 10mA to 100mA Schottky diode or signal diode such
as a 1N4148 works well for D2 in most applications. If
the input voltage can go below 6V, use a Schottky
diode for slightly improved efficiency and dropout char-
a c te ris tic s . Don’t us e la rg e p owe r d iod e s s uc h a s
1N5817 or 1N4001, since high junction capacitance
can cause VL to be pumped up to excessive voltages.
Low input voltages and low input-output differential volt-
ages each require some extra care in the design. Low
absolute input voltages can cause the VL linear regula-
tor to enter dropout, and eventually shut itself off. Low
input voltages relative to the output (low V -V
differ-
IN OUT
ential) can cause bad load regulation in multi-output fly-
back applications. See Transformer Design section.
Finally, low V -V
output voltage to sag when the load current changes
abruptly. The amplitude of the sag is a function of induc-
tor value and maximum duty factor (D
Characteristics parameter, 98% guaranteed over tem-
perature at f = 150kHz) as follows:
differentials can also cause the
IN OUT
Re c t ifie r Dio d e D3
(Tra n s fo rm e r S e c o n d a ry Dio d e )
The secondary diode in coupled-inductor applications
must withstand high flyback voltages greater than 60V,
whic h us ua lly rule s out mos t Sc hottky re c tifie rs .
Common silicon rectifiers such as the 1N4001 are also
prohibited, as they are far too slow. This often makes
fast silicon rectifiers such as the MURS120 the only
choice. The flyback voltage across the rectifier is relat-
an Electrical
MAX
2
(I
STEP
) x L
V
SAG
= ———————————————
2 x C
x (V
x D
- V
)
OUT
IN(MIN)
MAX
OUT
ed to the V -V
former turns ratio:
difference according to the trans-
IN OUT
The cure for low-voltage sag is to increase the value of
the output capacitor. For example, at V = 5.5V, V
IN
OUT
V
= V
+ (V - V
) x N
OUT
FLYBACK
SEC
IN
= 5V, L = 10µH, f = 150kHz, a total capacitance of
660µF will prevent excessive sag. Note that only the
capacitance requirement is increased and the ESR
re q uire me nts d on’t c ha ng e . The re fore , the a d d e d
c a p a c ita nc e c a n b e s up p lie d b y a low-c os t b ulk
capacitor in parallel with the normal low-ESR capacitor.
Table 4 summarizes low-voltage operational issues.
where: N is the transformer turns ratio SEC/PRI
V
V
OUT
is the maximum secondary DC output voltage
is the primary (main) output voltage
SEC
Subtract the main output voltage (V
) from V
OUT FLYBACK
in this equation if the secondary winding is returned to
and not to ground. The diode reverse breakdown
V
OUT
rating must also accommodate any ringing due to leak-
age inductance. D3’s current rating should be at least
twice the DC load current on the secondary output.
Table 4. Low-Voltage Troubleshooting
SOLUTION
SYMPTOM
CONDITION
ROOT CAUSE
Increase bulk output capacitance per
formula above. Reduce inductor value.
Sag or droop in V
under step load change
Low V -V
<1V
differential, Limited inductor-current slew
rate per cycle.
OUT
IN OUT
Dropout voltage is too
Reduce f to 150kHz. Reduce MOSFET
on-resistance and coil DCR.
Low V -V
<0.5V
differential, Maximum duty-cycle limits
exceeded.
IN OUT
high (V
follows V as
IN
OUT
V
IN
decreases)
Increase the minimum input voltage or
ignore.
Unstable—jitters between Low V -V
differential, Normal function of internal low-
dropout circuitry.
IN OUT
two distinct duty factors
<0.5V
Not enough duty cycle left to
differential, initiate forward-mode operation.
Reduce f to 150kHz. Reduce secondary
impedances—use Schottky if possible.
Stack secondary winding on main output.
Secondary output won’t
support a load
Low V -V
IN OUT
V
< 1.3 x V
(main)
Small AC current in primary can’t
store energy for flyback operation.
IN
OUT
Use a small 20mA Schottky diode for
boost diode D2. Supply VL from an
external source.
VL linear regulator is going into
dropout and isn’t providing
good gate-drive levels.
High supply current,
poor efficiency
Low input voltage, <5V
Low input voltage, <4.5V
Won’t start under load or
quits before battery is
completely dead
Supply VL from an external source other
VL output is so low that it hits the
VL UVLO threshold at 4.2V max.
than V , such as the system 5V supply.
BATT
______________________________________________________________________________________ 23
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
