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Operation Description (Continued)
LDO - Low Drop Out Operation
The LM3670 can operate at 100% duty cycle (no switching,
PMOS switch is completely on) for low drop out support of
the output voltage. In this way the output voltage is con-
trolled down to the lowest possible input voltage.
The minimum input voltage needed to support the output
voltage is
I
LOAD
Load current
R
DSON,
PFET
Drain to source resistance of PFET switch
in the triode region
R
INDUCTOR Inductor resistance
Application Information
OUTPUT VOLTAGE SELECTION FOR ADJUSTABLE
LM3670
The output voltage of the adjustable parts can be pro-
grammed through the resistor network connected from V
OUT
to V
FB then to GND. VOUT is adjusted to make VFB equal to
0.5V. The resistor from V
FB to GND (R2) should be at least
100K
to keep the current sunk through this network well
below the 15A quiescent current level (PFM mode with no
switching) but large enough that it is not susceptible to noise.
If R
2 is 200K
, and V
FB is 0.5V, then the current through the
resistor feedback network is 2.5A ( I
FB =0.5V/R2). The
output voltage formula is:
V
OUT Output Voltage (V)
V
FB Feedback Voltage (0.5V typ)
R
1 Resistor from VOUT to VFB (
)
R
2 Resistor from VOUT to GND (
)
For any output voltage greater than or equal to 0.7V a
frequency zero must be added at 10kHz for stability. The
formula is:
For any output voltages below 0.7 and above or equal to
2.5V, a pole must also be placed at 10kHz as well. The
lowest output voltage possible is 0.7V. At low output voltages
the duty cycle is very small and, as the input voltage in-
creases, the duty cycle decreases even further. Since the
duty cycle is so low any change due to noise is an appre-
ciable percentage. In other words, it is susceptible to noise.
Capacitors C
1 and C2 act as noise filters rather than fre-
quency poles and zeros. If the pole and zero are at the same
frequency the formula is:
A pole can also be used at higher output voltages. For
example, in the table Table 3, there is an entry for 1.24V with
both a pole and zero at approximately 10kHz for noise
rejection.
INDUCTOR SELECTION
There are two main considerations when choosing an induc-
tor; the inductor current should not saturate, and the inductor
current ripple is small enough to achieve the desired output
voltage ripple.
There are two methods to choose the inductor current rating.
Method 1:
The total current is the sum of the load and the inductor
ripple current. This can be written as
I
LOAD load current
V
IN input voltage
L inductor
f switching frequency
I
RIPPLE peak-to-peak
Method 2:
A more conservative approach is to choose an inductor that
can handle the current limit of 700 mA.
Given a peak-to-peak current ripple (I
PP) the inductor needs
to be at least
A 10 H inductor with a saturation current rating of at least
800 mA is recommended for most applications. The induc-
tor’s resistance should be less than around 0.3
for good
efficiency. Table 1 lists suggested inductors and suppliers.
For low-cost applications, an unshielded bobbin inductor is
suggested. For noise critical applications, a toroidal or
shielded-bobbin inductor should be used. A good practice is
to lay out the board with overlapping footprints of both types
for design flexibility. This allows substitution of a low-noise
toroidal inductor, in the event that noise from low-cost bobbin
models is unacceptable.
INPUT CAPACITOR SELECTION
A ceramic input capacitor of 4.7 F is sufficient for most
applications. A larger value may be used for improved input
voltage filtering. The input filter capacitor supplies current to
the PFET switch of the LM3670 in the first half of each cycle
and reduces voltage ripple imposed on the input power
source. A ceramic capcitor’s low ESR provides the best
LM3670
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相关PDF资料
LM368H-6.2 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 6.2 V, MBCY8
LM368YH-6.2 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 6.2 V, MBCY8
LM385AXM-2.5X 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 2.5 V, PDSO8
LM285AM-2.5X 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 2.5 V, PDSO8
LM385AYM-2.5X 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 2.5 V, PDSO8
LM385AM-2.5X 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 2.5 V, PDSO8
LM385D-2.5R2 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 2.5 V, PDSO8
LM385D-2.5R1 1-OUTPUT TWO TERM VOLTAGE REFERENCE, 2.5 V, PDSO8
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