AIC1577PTR 查看數據表（PDF） - Analog Intergrations
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AIC1577PTR Datasheet PDF : 14 Pages
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AIC1577
Output Diode Selection
In order not to exceed the diode ratings, it is impor-
tant to specify the diode peak current and average
power dissipation.
CIN and COUT Selection
To prevent the high voltage spike resulted from high
frequency switching, a low ESR input capacitor for
the maximum RMS current must be used. Usually
capacitors may be paralleled to meet size or height
requirements in the design.
The selection of COUT depends on the required ef-
fective series resistance (ESR). In general once the
ESR requirement is met, the capacitance is suitable
for filtering. The output ripple voltage (ΔVOUT) is
determined by:
∆VOUT
≈
∆IL ESR
+
1
4fCOUT
where f = operating frequency, COUT = output ca-
pacitance and ΔIL = ripple current of the inductor.
Once the ESR requirement for COUT has been met,
the RMS current rating generally far exceeds the
IRIPPLE(P-P) requirement.
Topside MOSFET Driver Supply (C3)
External bootstrap capacitor C3 connecting to
BOOST pin supplies the gate drive voltage for high-
side MOSFET. C3 is charged from INTVCC when
SW pin is low. When the high-side MOSFET turns
on, the driver places the C3 voltage across the gate
to the source of MOSFET. It will enhance the
MOSFET and turn on the high-side switch. Then
the switch node voltage SW rises to VIN and
BOOST pin rises to VIN + INTVCC. In general,
0.1µF is acceptable.
Output Voltage Programming
The typical AIC1577 application circuit is shown in
figure18. A resistive divider, as in the following for-
mula, sets the output voltage.
VOUT
=
0.8V1 +
R2
R1
The feedback reference voltage 0.8V allows low
output voltages from 0.8V to input voltage. A small
capacitor at 1nF in parallel to the upper feedback
resistor is required for a stable feedback.
ITH/RUN Function
The ITH/RUN pin, also as a dual-purpose pin, pro-
vides loop compensation as well as shutdown func-
tion. An internal current source at 2.5µA charges up
the external capacitor C5. When the voltage on
ITH/RUN pin reaches 0.8V, the AIC1577 begins to
operate.
VIN 4.5V~24V
R4
1.2M
D2
ITH/RUN
C7
1µF
LL4148
C5
330pF
R3
24k
Fig. 12 ITH/RUN pin interfacing
Over Current Protection
Over current protection occurs when the peak in-
ductor current reaches maximum current sense
threshold divided by sense resistor. The maximum
current under over current protection can be calcu-
lated by the following formula.
150mV(Maximum current sense threshold)
IMAX =
RS
At the same time, the frequency of oscillator will be
reduced to sixteenth of original value, 500kHz. This
lower frequency allows the inductor current to
safely discharge, thereby preventing current run-
away. The frequency of oscillator will automatically
10
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