LTC3809-1_15 Datasheet, PDF (20/24 Pages) Linear Technology – No RSENSE, Low Input Voltage, Synchronous DC/DC Controller with Output Tracking

LTC3809-1
APPLICATIONS INFORMATION
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. 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 ΔV_SENSE(MAX) is approximately 125mV.
Maximum Duty Cycle = ^VOUT = 65.5%
V_IN(MIN)
From Figure 1, SF = 82%.
R_DS(ON)MAX
=
5
6
•
0.9
•
SF
•
ΔV_SENSE(MAX)
I_OUT(MAX) • ρ_T
=
0.032Ω
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_SC
=
90mV
0.017Ω
=
5.3A
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_MIN
=
1.8V
550kHz • 600mA
•
⎛
⎝⎜^1−
1.8V ^⎞
2.75V ⎠⎟
=
1.88μH
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.
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