參數(shù)資料
型號(hào)： LTC3809EMS#PBF
廠商： LINEAR TECHNOLOGY CORP
元件分類(lèi)： 穩(wěn)壓器
英文描述： 1 A SWITCHING CONTROLLER, 825 kHz SWITCHING FREQ-MAX, PDSO10
封裝： PLASTIC, MSOP-10
文件頁(yè)數(shù)： 13/24頁(yè)
文件大小： 266K
代理商： LTC3809EMS#PBF
20
LTC3809
3809f
equal to (
ILOAD)(ESR),whereESRistheeffectiveseries
resistance of COUT. ILOAD also begins to charge or
discharge COUT generating a feedback error signal used by
the regulator to return VOUT to its steady-state value.
During this recovery time, VOUT can be monitored for
overshoot or ringing that would indicate a stability prob-
lem. OPTI-LOOP compensation allows the transient re-
sponse to be optimized over a wide range of output
capacitance and ESR values.
The ITH series RC-CC filter (see Functional Diagram) sets
the dominant pole-zero loop compensation.
The ITH external components showed in the figure on the
first page of this data sheet will provide adequate compen-
sation for most applications. The values can be modified
slightly (from 0.2 to 5 times their suggested values) to
optimize transient response once the final PC layout is
done and the particular output capacitor type and value
have been determined. The output capacitor needs to be
decided upon because the various types and values deter-
mine the loop feedback factor gain and phase. An output
current pulse of 20% to 100% of full load current having
a rise time of 1
s to 10s will produce output voltage and
ITH pin waveforms that will give a sense of the overall loop
stability. The gain of the loop will be increased by increas-
ing RC and the bandwidth of the loop will be increased by
decreasing CC. The output voltage settling behavior is
related to the stability of the closed-loop system and will
demonstrate the actual overall supply performance. For a
detailed explanation of optimizing the compensation com-
ponents, including a review of control loop theory, refer to
Application Note 76.
A second, more severe transient is caused by switching in
loads with large (>1
F) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with COUT, causing a rapid drop in VOUT. No regulator can
deliver enough current to prevent this problem if the load
switch resistance is low and it is driven quickly. The only
solution is to limit the rise time of the switch drive so that
the load rise time is limited to approximately (25) (CLOAD).
Thus a 10
F capacitor would be require a 250s rise time,
limiting the charging current to about 200mA.
APPLICATIO S I FOR ATIO
WU
UU
Design Example
As a design example, assume VIN will be operating from a
maximum of 4.2V down to a minimum of 2.75V (powered
by a single lithium-ion battery). Load current requirement
is a maximum of 2A, but most of the time it will be in a
standby mode requiring only 2mA. Efficiency at both low
and high load currents is important. Burst Mode operation
at light loads is desired. Output voltage is 1.8V. The IPRG
pin will be left floating, so the maximum current sense
threshold
VSENSE(MAX) is approximately 125mV.
MaximumDuty Cycle
V
OUT
IN MIN
=
()
.%
= 65 5
From Figure 1, SF = 82%.
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
T
()
.
.
=
=
5
6
09
0032
ρ
A 0.032
P-channel MOSFET in Si7540DP is close to this
value.
The N-channel MOSFET in Si7540DP has 0.017
RDS(ON).
The short circuit current is:
I
mV
A
SC =
=
90
0 017
53
.
So the inductor current rating should be higher than 5.3A.
The PLLLPF pin will be left floating, so the LTC3809 will
operate at its default frequency of 550kHz. For continuous
Burst Mode operation with 600mA IRIPPLE, the required
minimum inductor value is:
L
V
kHz
mA
V
H
MIN =
=
18
550
600
1
18
275
188
.
.
A 6A 2.2
H inductor works well for this application.
CIN will require an RMS current rating of at least 1A at
temperature. A COUT with 0.1 ESR will cause approxi-
mately 60mV output ripple.
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