MAX1889 データシートの表示（PDF） - Maxim Integrated
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MAX1889 Datasheet PDF : 32 Pages
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Triple-Output TFT LCD Power Supply
with Fault Protection
Inductor Selection
The minimum inductance value, peak current rating,
series resistance, and size are factors to consider when
selecting the inductor. These factors influence the con-
verter’s efficiency, maximum output load capability,
transient response time, and output voltage ripple. For
most applications, values between 3.3µH and 20µH
work best with the MAX1889’s switching frequencies.
The maximum load current, input voltage, output volt-
age, and switching frequency determine the inductor
value. For a given load current, higher inductor value
results in lower peak current and, thus, less output rip-
ple, but degrades the transient response and possibly
increases the size of the inductor. The equations pro-
vided here include a constant defined as LIR, which is
the ratio of the peak-to-peak inductor current ripple to
the average DC inductor current. For a good compro-
mise between the size of the inductor, power loss, and
output voltage ripple, select an LIR of 0.3 to 0.5. The
inductance value is then given by:
L
=
VIN(TYP)
VMAIN
2
VMAIN - VIN(TYP)
IMAIN(MAX)fOSC
1
LIR
η
where η is the efficiency, fOSC is the oscillator frequency
(see the Electrical Characteristics), and IMAIN includes
the primary load current and the input supply currents
for the charge pumps. Considering the typical applica-
tion circuit, the maximum average DC load current
(IMAIN(MAX)) is 200mA with a 9V output. Based on the
above equations, and assuming 85% efficiency and a
switching frequency of 1MHz, the inductance value is
9.4µH for an LIR of 0.3. The inductance value is 5.6µH
for an LIR of 0.5. The inductance in the standard appli-
cation circuit is chosen to be 6.8µH.
The inductor’s peak current rating should be higher than
the peak inductor current throughout the normal operat-
ing range. The peak inductor current is given by:
IPEAK
=
IMAIN(MAX)VMAIN
VIN(MIN)
1+
LIR
2
1
η
Under fault conditions, the inductor current can reach the
internal LX current limit (see the Electrical Characteristics).
However, soft saturation inductors and the controller’s fast
current-limit circuitry protect the device from failure during
such a fault condition.
The inductor’s DC resistance can significantly affect
efficiency due to conduction losses in the inductor.
The power loss due to the inductor’s series resistance
(PLR) can be approximated by the following equation:
PLR
=
I(LAVG)2RL
≅
IMAIN × VMAIN
VIN
2
RL
where IL(AVG) is the average inductor current and RL is
the inductor’s series resistance. For best performance,
select inductors with resistance less than the internal
N-channel MOSFET’s on-resistance (0.25Ω typ). To
minimize radiated noise in sensitive applications, use a
shielded inductor.
Output Capacitor
The output capacitor affects the circuit stability and out-
put voltage ripple. A 10µF ceramic capacitor works well
in most applications. Depending on the output capaci-
tor chosen, feedback compensation may be required
or desirable to increase the loop-phase margin or
increase the loop bandwidth for transient response
(see the Feedback Compensation section).
The total output voltage ripple has two components: the
capacitive ripple caused by the charging and discharg-
ing of the output capacitance, and the ohm ripple due to
the capacitor’s equivalent series resistance (ESR):
VRIPPLE = VRIPPLE(ESR) + VRIPPLE(C)
VRIPPLE(ESR) ≈ IPEAKRESR(COUT), and
VRIPPLE(C)
≈
IMAIN
COUT
VMAIN - VIN
VMAINfOSC
where IPEAK is the peak inductor current (see the Inductor
Selection section). For ceramic capacitors, the output volt-
age ripple is typically dominated by VRIPPLE(C). The volt-
age rating and temperature characteristics of the output
capacitor must also be considered.
Step-Up Regulator Compensation
The loop stability of a current-mode step-up regulator
can be analyzed using a small-signal model. In continu-
ous conduction mode (CCM), the loop-gain transfer
function consists of a dominant pole, a high-frequency
pole, a right-half-plane (RHP) zero, and an ESR zero. In
the case of ceramic output capacitors, the ESR zero is at
a very high frequency.
______________________________________________________________________________________ 17
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