ISL97642 데이터 시트보기 (PDF) - Renesas Electronics
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ISL97642 Datasheet PDF : 19 Pages
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ISL97642
Table 2 gives typical values (margins are considered 10%, 3%,
20%, 10% and 15% on VIN, VO, L, fS and ILMT:
TABLE 2.
VIN (V)
3.3
VO (V)
9
L (µH)
6.8
fS (MHz)
1.2
IOMAX (mA)
890
3.3
12
6.8
1.2
666
3.3
15
6.8
1.2
530
5
9
6.8
1.2
1350
5
12
6.8
1.2
1000
5
15
6.8
1.2
795
Input Capacitor
The input capacitor is used to supply the current to the
converter. It is recommended that CIN be larger than 10F. The
reflected ripple voltage will be smaller with larger CIN. The
voltage rating of input capacitor should be larger than the
maximum input voltage.
Boost Inductor
The boost inductor is a critical part which influences the output
voltage ripple, transient response, and efficiency. Value of
3.3H to 10H inductor is recommended in applications to fit
the internal slope compensation. The inductor must be able to
handle the following average and peak current:
ILPK = ILAVG + ---2--I-L--
ILAVG = 1----I-–-O----D---
(EQ. 5)
Rectifier Diode
A high-speed diode is desired due to the high switching
frequency. Schottky diodes are recommended because of their
fast recovery time and low forward voltage. The rectifier diode
must meet the output current and peak inductor current
requirements.
Output Capacitor
The output capacitor supplies the load directly and reduces the
ripple voltage at the output. Output ripple voltage consists of
two components: the voltage drop due to the inductor ripple
current flowing through the ESR of output capacitor, and the
charging and discharging of the output capacitor.
VRIPPLE = ILPK ESR + -V----O---V--–---O--V----I--N-- C-----O-I--O--U----T- -f-1S--
(EQ. 6)
For low ESR ceramic capacitors, the output ripple is dominated
by the charging and discharging of the output capacitor. The
voltage rating of the output capacitor should be greater than
the maximum output voltage.
NOTE: Capacitors have a voltage coefficient that makes their effective
capacitance drop as the voltage across them increases. COUT in the
FN6436 Rev 0.00
June 18, 2007
Equation 6 assumes the effective value of the capacitor at a particular
voltage and not the manufacturer’s stated value, measured at zero
volts.
Compensation
The ISL97642 incorporates a transconductance amplifier in its
feedback path to allow the user some adjustment on the
transient response and better regulation. The ISL97642 uses
current mode control architecture, which has a fast current
sense loop and a slow voltage feedback loop. The fast current
feedback loop does not require any compensation. The slow
voltage loop must be compensated for stable operation. The
compensation network is a series RC network from COMP pin
to ground. The resistor sets the high frequency integrator gain
for fast transient response and the capacitor sets the integrator
zero to ensure loop stability. For most applications, a 2.2nF
capacitor and a 180 resistor are inserted in series between
COMP pin and ground. To improve the transient response,
either the resistor value can be increased or the capacitor
value can be reduced, but too high resistor value or too low
capacitor value will reduce loop stability.
Boost Feedback Resistors
As the boost output voltage, VBOOST, is reduced below 12V,
the effective voltage feedback in the IC increases the ratio of
voltage to current feedback at the summing comparator
because R2 decreases relative to R1. To maintain stable
operation over the complete current range of the IC, the
voltage feedback to the FBB pin should be reduced
proportionally (as VBOOST is reduced) by means of a series
resistor-capacitor network (R7 and C7) in parallel with R1, with
a pole frequency (fp) set to approximately 10kHz for C2
(effective) = 10µF and 4kHz for C2 (effective) = 30µF.
R7 = 1 0.1 R2 – 1 R1^-1
(EQ. 7)
C7 = 1 2 3.142 fp R7
(EQ. 8)
Linear-Regulator Controllers (VON and VOFF)
The ISL97642 includes 2 independent linear-regulator
controllers, in which there is one positive output voltage (VON)
and one negative voltage (VOFF). The VON and VOFF linear-
regulator controller function diagram, application circuit and
waveforms are shown in Figures 18 and 19 respectively.
Page 12 of 19
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