ISL97646 Просмотр технического описания (PDF) - Intersil
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ISL97646 Datasheet PDF : 15 Pages
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ISL97646
Operation
The boost converter is a current mode PWM converter
operating at either a 650kHz or 1.2MHz. It can operate in
both discontinuous conduction mode (DCM) at light load 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:
V-----B----O----O-----S----T-
VIN
=
------1-------
1–D
(EQ. 1)
Where D is the duty cycle of the switching MOSFET.
Figure 13 shows the block diagram of the boost regulator. It
uses a summing amplifier architecture consisting of gm
stages 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 reached.
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 60kΩ is recommended.
The boost converter output voltage is determined by
Equation 2:
VBOOST
=
R-----1-----+----R-----2-
R2
×
VF
B
(EQ. 2)
The current through the MOSFET is limited to 2.6APEAK.
This restricts the maximum output current (average) based
on the Equation 3:
IOMAX
=
⎛
⎝
IL
M
T
–
-Δ--2--I--L-⎠⎞
×
V-----I--N--
VO
(EQ. 3)
Where ΔIL is peak to peak inductor ripple current, and is set
by:
ΔIL
=
-V----I--N--
L
×
-D--
fs
(EQ. 4)
where fS is the switching frequency (650kHz or 1.2MHz).
Table 2 gives typical values (margins are considered 10%,
3%, 20%, 10% and 15% on VIN, VO, L, fS and IOMAX).
Capacitor
An input capacitor is used to suppress the voltage ripple
injected into the boost converter. The 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 1
for input capacitor.
TABLE 1. BOOST CONVERTER INPUT CAPACITOR
RECOMMENDATION
CAPACITOR SIZE MFG
PART NUMBER
10µF/16V
1206 TDK
C3216X7R1C106M
10µF/10V
0805 Murata GRM21BR61A106K
22µF/10V
1210 Murata GRB32ER61A226K
VIN (V)
3
3
3
5
5
5
5
3
3
3
5
5
5
5
TABLE 2. MAXIMUM OUTPUT CURRENT CALCULATION
VO (V)
9
L (µH)
10
fs (MHz)
0.65
12
10
0.65
15
10
0.65
9
10
0.65
12
10
0.65
15
10
0.65
18
10
0.65
9
10
1.2
12
10
1.2
15
10
1.2
9
10
1.2
12
10
1.2
15
10
1.2
18
10
1.2
IOMAX (mA)
636
419
289
1060
699
482
338
742
525
395
1236
875
658
514
10
FN6265.1
December 14, 2007
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