CS51033GDR8(2002) データシートの表示（PDF） - ON Semiconductor
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CS51033GDR8 Datasheet PDF : 12 Pages
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CS51033
TON(MAX) + 5.0 ms 0.53 + 2.65 ms
TON(MIN) + 5.0 ms 0.35 + 1.75 ms
TOFF(MAX) + 5.0 ms * 0.7 ms + 4.3 ms
3) Inductor Selection
Pick the inductor value to maintain continuous mode
operation down to 0.3 Amps.
The ripple current ∆I = 2 × IOUT(MIN) = 2 × 0.3 A = 0.6 A.
LMIN
+
VOUT
)
VD
DI
TOFF(MAX)
+
2.1
V
0.6
4.3
A
ms
^
15
mH
The CS51033 will operate with almost any value of
inductor. With larger inductors the ripple current is reduced
and the regulator will remain in a continuous conduction
mode for lower values of load current. A smaller inductor
will result in larger ripple current. The core must not saturate
with the maximum expected current, here given by:
IMAX
+
IOUT )
2.0
DI
+
3.0
A
)
0.6
Ań2.0
+
3.3
A
4) Output Capacitor
The output capacitor limits the output ripple voltage. The
CS51033 needs a maximum of 15 mV of output ripple for
the feedback comparator to change state. If we assume that
all the inductor ripple current flows through the output
capacitor and that it is an ideal capacitor (i.e. zero ESR), the
minimum capacitance needed to limit the output ripple to
50 mV peak to peak is given by:
CO + 8.0
DI
FSW
DV
+
8.0
(200
0.6 A
103 Hz) (33
10*3 V) ^ 11.4 mF
The minimum ESR needed to limit the output voltage
ripple to 50 mV peak to peak is:
ESR
+
DV
DI
+
50
10*3
0.6 A
+
55
mW
The output capacitor should be chosen so that its ESR is
at least half of the calculated value and the capacitance is at
least ten times the calculated value. It is often advisable to
use several capacitors in parallel to reduce ESR.
Low impedance aluminum electrolytic, tantalum or
organic semiconductor capacitors are a good choice for an
output capacitor. Low impedance aluminum are the
cheapest but are not available in surface mount at present.
Solid tantalum chip capacitors are available from a number
of suppliers and offer the best choice for surface mount
applications. The capacitor working voltage should be
greater than the output voltage in all cases.
5) VFB Divider
ǒ Ǔ ǒ Ǔ VOUT +
1.25 V
R1 ) R2
R2
+ 1.25 V
R1
R2
)
1.0
The input bias current to the comparator is 4.0 µA. The
resistor divider current should be considerably higher than
this to ensure that there is sufficient bias current. If we
choose the divider current to be at least 250 times the bias
current this gives a divider current of 1.0 mA and simplifies
the calculations.
1.5 V
1.0 mA
+
R1
)
R2
+
1.5
kW
Let R2 = 1.0 k
Rearranging the divider equation gives:
ǒ Ǔ ǒ Ǔ R1 + R2
VOUT
1.25
*
1.0
+ 1.0 kW
1.5 V
1.25
+ 200 W
6) Divider Bypass Capacitor CRR
Since the feedback resistors divide the output voltage by
a factor of 4.0, i.e. 5.0 V/1.25 V= 4.0, it follows that the
output ripple is also divided by four. This would require that
the output ripple be at least 60 mV (4.0 × 15 mV) to trip the
feedback comparator. We use a capacitor CRR to act as an
AC short so that the output ripple is not attenuated by the
divider network. The ripple voltage frequency is equal to the
switching frequency so we choose CRR so that:
XC
+
1.0
2pfC
is negligible at the switching frequency.
In this case FSW is 200 kHz if we allow XC = 3.0 Ω then:
C
+
1.0
2pf3
^
0.265
mF
7) Soft Start and Fault Timing Capacitor CS
CS performs several important functions. First it provides
a dead time for load transients so that the IC does not enter
a fault mode every time the load changes abruptly. Secondly
it disables the fault circuitry during startup, it also provides
Soft Start by clamping the reference voltage during startup
to rise slowly and finally it controls the hiccup short circuit
protection circuitry. This function reduces the PFET’s duty
cycle to 2.0% of the CS period.
The most important consideration in calculating CS is that
it’s voltage does not reach 2.5 V (the voltage at which the
fault detect circuitry is enabled) before VFB reaches 1.15 V
otherwise the power supply will never start.
If the VFB pin reaches 1.15 V, the fault timing comparator
will discharge CS and the supply will not start. For the VFB
voltage to reach 1.15 V the output voltage must be at least
4 × 1.15 = 4.6 V.
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