Datasheet

LTC3809-1
20
38091fc
pulse of 20% to 100% of full load current having a rise
time of 1μs to 10μs will produce output voltage and I
TH
pin waveforms that will give a sense of the overall loop
stability. The gain of the loop will be increased by increas-
ing R
C
and the bandwidth of the loop will be increased
by decreasing C
C
. 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
components, 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 (>1μF) supply bypass capacitors. The
discharged bypass capacitors are effectively put in parallel
with C
OUT
, causing a rapid drop in V
OUT
. 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) • (C
LOAD
).
Thus a 10μF capacitor would be require a 250μs rise time,
limiting the charging current to about 200mA.
Design Example
As a design example, assume V
IN
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. Effi ciency 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 fl oating, so the maximum current sense
threshold ΔV
SENSE(MAX)
is approximately 125mV.
Maximum Duty Cycle
V
V
OUT
IN MIN
=
()
.%= 65 5
From Figure 1, SF = 82%.
RSF
V
I
DS ON MAX
SENSE MAX
OUT MAX
()
()
()
•.• •
•
=
Δ
5
6
09
ρρ
T
=Ω0 032.
APPLICATIONS INFORMATION
A 0.032Ω P-channel MOSFET in Si7540DP is close to
this value.
The N-channel MOSFET in Si7540DP has 0.017Ω R
DS(ON)
.
The short-circuit current is:
I
mV
A
SC
=
Ω
=
90
0 017
53
.
.
So the inductor current rating should be higher than 5.3A.
The LTC3809-1 operates at a frequency of 550kHz. For
continuous Burst Mode operation with 600mA I
RIPPLE
,
the required minimum inductor value is:
L
V
kHz mA
V
V
H
MIN
=−
⎛
⎝
⎜
⎞
⎠
⎟
=μ
18
550 600
1
18
275
188
.
•
•
.
.
.
A 6A 2.2μH inductor works well for this application.
C
IN
will require an RMS current rating of at least 1A
at temperature. A C
OUT
with 0.1Ω ESR will cause
approximately 60mV output ripple.
PC Board Layout Checklist
When laying out the printed circuit board, use the following
checklist to ensure proper operation of the LTC3809-1.
• The power loop (input capacitor, MOSFET, inductor,
output capacitor) should be as small as possible and
isolated as much as possible from LTC3809-1.
• Put the feedback resistors close to the V
FB
pins. The I
TH
compensation components should also be very close
to the LTC3809-1.
• The current sense traces should be Kelvin connections
right at the P-channel MOSFET source and drain.
• Keeping the switch node (SW) and the gate driver nodes
(TG, BG) away from the small-signal components,
especially the feedback resistors, and I
TH
compensation
components.