MAX1652 Datasheet, PDF (23/28 Pages) Maxim Integrated Products – High-Efficiency, PWM, Step-Down DC-DC Controllers in 16-Pin QSOP

High-Efficiency, PWM, Step-Down
DC-DC Controllers in 16-Pin QSOP
Boost-Supply Diode D2
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-
acteristics. Don’t use large power diodes such as
1N5817 or 1N4001, since high junction capacitance
can cause VL to be pumped up to excessive voltages.
Rectifier Diode D3
(Transformer Secondary Diode)
The secondary diode in coupled-inductor applications
must withstand high flyback voltages greater than 60V,
which usually rules out most Schottky rectifiers.
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-
ed to the V_IN-V_OUT difference according to the trans-
former turns ratio:
V_FLYBACK = V_SEC + (V_IN - V_OUT) x N
where: N is the transformer turns ratio SEC/PRI
V_SEC is the maximum secondary DC output voltage
V_OUT is the primary (main) output voltage
Subtract the main output voltage (V_OUT) from V_FLYBACK
in this equation if the secondary winding is returned to
V_OUT and not to ground. The diode reverse breakdown
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.
_____________Low-Voltage Operation
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_IN-V_OUT differ-
ential) can cause bad load regulation in multi-output fly-
back applications. See Transformer Design section.
Finally, low V_IN-V_OUT differentials can also cause the
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_MAX an Electrical
Characteristics parameter, 98% guaranteed over tem-
perature at f = 150kHz) as follows:
(I_STEP)² x L
V_SAG = ———————————————
2 x C_OUT x (V_IN(MIN) x D_MAX - V_OUT)
The cure for low-voltage sag is to increase the value of
the output capacitor. For example, at V_IN = 5.5V, V_OUT
= 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
requirements don’t change. Therefore, the added
capacitance can be supplied by a low-cost bulk
capacitor in parallel with the normal low-ESR capacitor.
Table 4 summarizes low-voltage operational issues.
Table 4. Low-Voltage Troubleshooting
SYMPTOM
CONDITION
ROOT CAUSE
SOLUTION
Sag or droop in V_OUT
Low V_IN-V_OUT differential, Limited inductor-current slew Increase bulk output capacitance per
under step load change <1V
rate per cycle.
formula above. Reduce inductor value.
Dropout voltage is too
high (V_OUT follows V_IN as
V_IN decreases)
Low V_IN-V_OUT differential, Maximum duty-cycle limits
<0.5V
exceeded.
Reduce f to 150kHz. Reduce MOSFET
on-resistance and coil DCR.
Unstable—jitters between Low V_IN-V_OUT differential, Normal function of internal low- Increase the minimum input voltage or
two distinct duty factors <0.5V
dropout circuitry.
ignore.
Secondary output won’t
support a load
High supply current,
poor efficiency
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 can’t
store energy for flyback operation.
Reduce f to 150kHz. Reduce secondary
impedances—use Schottky if possible.
Stack secondary winding on main output.
Low input voltage, <5V
VL linear regulator is going into Use a small 20mA Schottky diode for
dropout and isn’t providing
boost diode D2. Supply VL from an
good gate-drive levels.
external source.
Won’t start under load or
quits before battery is
completely dead
Low input voltage, <4.5V
VL output is so low that it hits the Supply VL from an external source other
VL UVLO threshold at 4.2V max. than VBATT, such as the system 5V supply.
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