Rainbow Electronics MAX1635 User Manual
Page 21
MAX1630–MAX1635
Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
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21
where: on-state voltage drop V
Q_
= I
LOAD
x R
DS(ON)
C
RSS
= MOSFET reverse transfer capacitance
I
GATE
= DH driver peak output current capabil-
ity (1A typical)
20ns = DH driver inherent rise/fall time
Under output short-circuit, the MAX1633/MAX1634/
MAX1635’s synchronous rectifier MOSFET suffers extra
stress because its duty factor can increase to greater
than 0.9. It may need to be oversized to tolerate a con-
tinuous DC short circuit. During short circuit, the
MAX1630/MAX1631/MAX1632’s output undervoltage
shutdown protects the synchronous rectifier under out-
put short-circuit conditions.
To reduce EMI, add a 0.1µF ceramic capacitor from the
high-side switch drain to the low-side switch source.
Rectifier Clamp Diode
The rectifier is a clamp across the low-side MOSFET
that catches the negative inductor swing during the
60ns dead time between turning one MOSFET off and
each low-side MOSFET on. The latest generations of
MOSFETs incorporate a high-speed silicon body diode,
which serves as an adequate clamp diode if efficiency
is not of primary importance. A Schottky diode can be
placed in parallel with the body diode to reduce the for-
ward voltage drop, typically improving efficiency 1% to
2%. Use a diode with a DC current rating equal to one-
third of the load current; for example, use an MBR0530
(500mA-rated) type for loads up to 1.5A, a 1N5819 type
for loads up to 3A, or a 1N5822 type for loads up to
10A. The rectifier’s rated reverse breakdown voltage
must be at least equal to the maximum input voltage,
preferably with a 20% derating factor.
Boost-Supply Diode D2
A signal diode such as a 1N4148 works well in most
applications. If the input voltage can go below +6V, use
a small (20mA) Schottky diode for slightly improved
efficiency and dropout characteristics. Don’t use large
power diodes, such as 1N5817 or 1N4001, since high
junction capacitance can pump up VL to excessive
voltages.
Rectifier Diode D3
(Transformer Secondary Diode)
The secondary diode in coupled-inductor applications
must withstand flyback voltages greater than 60V,
which usually rules out most Schottky rectifiers.
Common silicon rectifiers, such as the 1N4001, are also
prohibited because they are 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:
where: N = the transformer turns ratio SEC/PRI
V
SEC
= the maximum secondary DC output
voltage
V
OUT
= 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
voltages each require extra care in their 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
dif-
ferential) can cause bad load regulation in multi-output
flyback applications (see the design equations in the
Transformer Design
section). Also, low V
IN
-V
OUT
differ-
entials can also cause the output voltage to sag when
the load current changes abruptly. The amplitude of the
sag is a function of inductor value and maximum duty
factor (an
Electrical Characteristics
parameter, 98%
guaranteed over temperature at f = 200kHz), as follows:
The cure for low-voltage sag is to increase the output
capacitor’s value. For example, at V
IN
= +5.5V, V
OUT
=
+5V, L = 10µH, f = 200kHz, I
STEP
= 3A, a total capaci-
tance of 660µF keeps the sag less than 200mV. Note
that only the capacitance requirement increases, 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.
V
=
(I
) x L
2 x C
x (V
x D
- V
)
SAG
STEP
2
OUT
IN(MAX)
MAX
OUT
V
= V
+ (V
- V
) x N
FLYBACK
SEC
IN
OUT
PD(upper FET) = (I
) x R
x DUTY
+ V x I
x f x
V x C
I
20ns
PD(lower FET) = (I
) x R
x (1 - DUTY)
DUTY = (V
+ V
) / (V
- V
)
LOAD
2
DS(ON)
IN
LOAD
IN
RSS
GATE
LOAD
2
DS(ON)
OUT
Q2
IN
Q1
+