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參數(shù)資料
| 型號： | MAX1802 |
| 廠商： | Maxim Integrated Products, Inc. |
| 英文描述： | Digital Camera Step-Down Power Supply |
| 中文描述： | 數(shù)碼相機降壓型電源 |
| 文件頁數(shù)： | 20/28頁 |
| 文件大小： | 469K |
| 代理商： | MAX1802 |
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M
Digital Camera Step-Down
Power Supply
20
______________________________________________________________________________________
Use discontinuous current when the step-up ratio
(V
OUT
/ V
IN
) is greater than 1 / (1 - D
MAX
).
Continuous Inductor Current
A reasonable inductor value (L
IDEAL
) can be derived
from the following equation, which sets continuous
peak-to-peak inductor current at 1/3 the DC inductor
current:
(
where D, the duty cycle, is given by:
In these equations, V
DSN
is the voltage drop across the
N-channel MOSFET switch, and V
D
is the forward volt-
age drop across the rectifier. Given L
IDEAL
, the consis-
tent peak-to-peak inductor current is 0.33 I
OUT
/ (1 - D).
The maximum inductor current is 1.17 I
OUT
/ (1 - D).
Inductance values smaller than L
IDEAL
can be used;
however, the maximum inductor current will rise as L is
reduced, and a larger output capacitance will be
required to maintain the same output ripple.
The inductor current will become discontinuous if I
OUT
decreases by more than a factor of six from the value
used to determine L
IDEAL
.
Discontinuous Inductor Current
In the discontinuous mode, each MAX1802 auxiliary
controller regulates the output voltage by adjusting the
duty cycle to allow adequate power transfer to the load.
To ensure regulation under worst-case load conditions
(maximum I
OUT
), choose:
The peak inductor current is V
IN
D
MAX
/ (L f
OSC
).
The inductor
’
s saturation current rating should meet or
exceed the calculated peak inductor current.
Input and Output Filter Capacitors
The input capacitor (C
IN
) reduces the current peaks
drawn from the battery or input power source. The
impedance of the input capacitor at the switching fre-
quency should be less than that of the input source so
that high-frequency switching currents do not pass
through the input source.
The output capacitor is required to keep the output volt-
age ripple small and to ensure regulation control-loop
stability. The output capacitor must have low imped-
ance at the switching frequency. Tantalum and ceramic
capacitors are good choices. Tantalum capacitors typi-
cally have high capacitance and medium-to-low equiv-
alent series resistance (ESR) so that ESR dominates the
impedance at the switching frequency. In turn, the out-
put ripple is approximately:
V
RIPPLE
≈
I
L(
p-p) ESR
where I
L
(p-p) is the peak-to-peak inductor current.
Ceramic capacitors typically have lower ESR than tan-
talum capacitors, but with relatively small capacitance
that dominates the impedance at the switching fre-
quency. In turn, the output ripple is approximately:
V
RIPPLE
≈
I
L(
p-p) Z
C
where I
L
(p-p) is the peak-to-peak inductor current, and
Z
C
≈
1 / (2
π
f
OSC
C
OUT
).
See the
Compensation Design
section for a discussion
of the influence of output capacitance and ESR on reg-
ulation control-loop stability.
The capacitor voltage rating must exceed the maximum
applied capacitor voltage. For most tantalum capaci-
tors, manufacturers suggest derating the capacitor by
applying no more than 70% of the rated voltage to the
capacitor. Ceramic capacitors are typically used up to
the voltage rating of the capacitor. Consult the manu-
facturer
’
s specifications for proper capacitor derating.
MOSFET Selection
The MAX1802 main converter and auxiliary controllers
drive external logic-level P- and/or N-channel MOSFETs
as the circuit switching elements. The key selection
parameters are:
On-resistance (R
DS(ON)
)
Maximum drain-to-source voltage (V
DS(MAX)
)
Total gate charge (Q
g
)
Reverse transfer capacitance (C
RSS
)
Because the main converter
’
s external MOSFETs are
used for current sense, they directly determine the out-
put current capability and efficiency of the main con-
verter. It is important to select the appropriate external
MOSFETs for the main converter. The P-channel on-
resistance (R
DSP
) at minimum input voltage (V
VDDM
)
must be low enough so that the converter can produce
the desired output current as determined by the
I
OUT(MAX)
equation in the
Main DC-DC Converter
sec-
tion. The N-channel on-resistance (R
DSN
) determines
L
V
2
D
I
OUT
OUT OSC
MAX
=
D
V
V
V
IN
OUT
D
≈
+
1
L
V
V
D
D
I
IDEAL
IN MAX
DSN
OUT OSC
=
)
(
)
)
3
1
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