ISL97653A Просмотр технического описания (PDF) - Renesas Electronics
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ISL97653A Datasheet PDF : 19 Pages
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ISL97653A
Application Information
AVDD Boost Converter
The AVDD boost converter features a fully integrated 4.4A
boost FET. The regulator uses a current mode PI control
scheme which provides good line regulation and good
transient response. It can operate in both discontinuous
conduction mode (DCM) at light loads and continuous mode
(CCM). In continuous current mode, current flows continuously
in the inductor during the entire switching cycle in steady state
operation. The voltage conversion ratio in continuous current
mode is given by Equation 1:
V-----bV---o-I--No----s---t = 1-----–-1----D---
where D is the duty cycle of the switching MOSFET.
(EQ. 1)
The boost soft-start function is digitally controlled within a fixed
10ms time frame during which the current limit is increased in
eight linear steps.
The boost converter uses a summing amplifier architecture for
voltage feedback, current feedback, and slope compensation.
A comparator looks at the peak inductor current cycle by cycle
and terminates the PWM cycle if the current limit is triggered.
Since this comparison is cycle based, the PWM output will be
released after the peak current goes below the current limit
threshold.
An external resistor divider is required to divide the output
voltage down to the nominal reference voltage. Current drawn
by the resistor network should be limited to maintain the overall
converter efficiency. The maximum value of the resistor
network is limited by the feedback input bias current and the
potential for noise being coupled into the feedback pin. A
resistor network in the order of 60k is recommended. The
boost converter output voltage is determined by Equation 2:
AVDD = R-----3--R---+--4--R-----4- VFBB
(EQ. 2)
where R3 and R4 are in the “Typical Application Diagram” on
page 5. Unless otherwise stated, component variables referred
to in equations refer to the Typical Application Diagram.
The current through the MOSFET is limited to 4.4A peak. This
restricts the maximum output current (average) based on
Equation 3:
IOMAX
=
IL
M
T
–
---2--I-L--
V-V----I-O-N--
(EQ. 3)
Where IL is peak to peak inductor ripple current, and is set by
Equation 4. fs is the switching frequency (680kHz).
IL = -V---L-I--N-- -fD-S--
(EQ. 4)
Table 1 gives typical values (worst case margins are
considered 10%, 3%, 20%, 10% and 15% on VIN, VO, L, FSW
and IOMAX):
TABLE 1. MAXIMUM OUTPUT CURRENT CALCULATION
VIN
VO
L
IOMAX
(V)
(V)
(µH)
(mA)
5
9
6.8
2215
5
12
6.8
1673
5
15
6.8
1344
12
15
6.8
3254
12
18
6.8
2670
Boost Converter Input Capacitor
An input capacitor is used to suppress the voltage ripple
injected into the boost converter. A ceramic capacitor with
capacitance larger than 10µF is recommended. The voltage
rating of input capacitor should be larger than the maximum
input voltage. Some capacitors are recommended in Table 2
for input capacitor.
TABLE 2. BOOST CONVERTER INPUT CAPACITOR
RECOMMENDATION
CAPACITOR SIZE
VENDOR
PART NUMBER
10µF/25V
10µF/25V
1210 TDK
1210 Murata
C3225X7R1E106M
GRM32DR61E106K
Boost Inductor
The boost inductor is a critical part which influences the output
voltage ripple, transient response, and efficiency. Values of
3.3µH to 10µH are recommended to match the internal slope
compensation as well as to maintain a good transient response
performance. The inductor must be able to handle the average
and peak currents expressed in Equations 5 and 6:
ILAVG = 1----I-–-O----D---
ILPK = ILAVG + ---2--I--L-
(EQ. 5)
(EQ. 6)
Some inductors are recommended in Table 3.
TABLE 3. BOOST INDUCTOR RECOMMENDATION
DIMENSIONS
INDUCTOR
(mm)
VENDOR
PART NUMBER
10µH/
5.1APEAK
5.9µH/
6APEAK
13x13x4.5 TDK
12.9X12.9X4 Sumida
RLF12545T-100M5R1
CDEP12D38NP-5R9MB-120
FN6367 Rev 3.00
September 7, 2010
Page 11 of 19
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