參數(shù)資料
型號(hào)： MAX1654
廠商： Maxim Integrated Products, Inc.
英文描述： High-Efficiency, PWM, Step-Down DC-DC Controllers in 16-Pin QSOP
中文描述： 高效率、PWM、降壓型DC-DC控制器，16引腳QSOP封裝
文件頁數(shù)： 23/28頁
文件大?。?/td> 265K
代理商： MAX1654
M
High-Effic ienc y, PWM, S tep-Down
DC-DC Controllers in 16-Pin QS OP
______________________________________________________________________________________
23
Boost-S upply 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.
Rec tifier Diode D3
(T ransformer S ec ondary 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
Supply VL from an external source other
than V
BATT
, such as the system 5V supply.
VL output is so low that it hits the
VL UVLO threshold at 4.2V max.
Low input voltage, <4.5V
Won’t start under load or
quits before battery is
completely dead
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.
Low input voltage, <5V
High supply current,
poor efficiency
Reduce f to 150kHz. Reduce secondary
impedances—use Schottky if possible.
Stack secondary winding on main output.
Not enough duty cycle left to
initiate forward-mode operation.
Small AC current in primary can’t
store energy for flyback operation.
Low V
IN
-V
OUT
differential,
V
IN
< 1.3 x V
OUT
(main)
Secondary output won’t
support a load
Increase the minimum input voltage or
ignore.
Normal function of internal low-
dropout circuitry.
Low V
IN
-V
OUT
differential,
<0.5V
Unstable—jitters between
two distinct duty factors
Reduce f to 150kHz. Reduce MOSFET
on-resistance and coil DCR.
Maximum duty-cycle limits
exceeded.
Low V
IN
-V
OUT
differential,
<0.5V
Dropout voltage is too
high (V
OUT
follows V
IN
as
V
IN
decreases)
Increase bulk output capacitance per
formula above. Reduce inductor value.
Limited inductor-current slew
rate per cycle.
Low V
IN
-V
OUT
differential,
<1V
Sag or droop in V
OUT
under step load change
SOLUTION
ROOT CAUSE
CONDITION
SYMPTOM
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MAX1654EEE+T 功能描述:DC/DC 開關(guān)控制器 PWM Step-Down RoHS:否 制造商:Texas Instruments 輸入電壓:6 V to 100 V 開關(guān)頻率: 輸出電壓:1.215 V to 80 V 輸出電流:3.5 A 輸出端數(shù)量:1 最大工作溫度:+ 125 C 安裝風(fēng)格: 封裝 / 箱體:CPAK
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