LTC3541
3541fa
PPLICATIO S I FOR ATIO
U
U
resistance of C
OUT
. 擨
LOAD
also begins to charge or dis-
charge C
OUT
, which generates a feedback error signal. The
regulator loop then acts to return V
OUT
to its steady-state
value. During this recovery time V
OUT
can be monitored
for overshoot or ringing that would indicate a stability
problem. For a detailed explanation of switching control
loop theory see Application Note 76.
A second, more severe transient is caused by switching
in loads with large (>1礔) supply bypass capacitors. The
discharged bypass capacitors are effectively put in paral-
lel 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 10礔 capacitor charging to 3.3V
would require a 250祍 rise time, limiting the charging
current to about 130mA.
VLDO/LINEAR REGULATOR
Adjustable Output Voltage
The LTC3541 LV
OUT
output voltage is set by the ratio of two
external resistors as shown in Figure 7. The device servos
LV
OUT
to maintain the LFB pin voltage at 0.4V (referenced
to ground). Thus, the current in R1 is equal to 0.4V/R1.
For good transient response, stability, and accuracy, the
current in R1 should be at least 2礎, thus the value of
R1 should be no greater than 200k. The current in R2 is
the current in R1 plus the LFB pin bias current. Since the
LFB pin bias current is typically <10nA, it can be ignored
in the output voltage calculation. The output voltage can
be calculated using the formula in Figure 8. Note that in
shutdown the output is turned off and the divider current
will be zero once C
OUT
is discharged.
The LTC3541 VLDO and linear regulator loops operate
at a relatively high gain of 3.5礦/mA and 3.4礦/mA
respectively, referred to the LFB input. Thus, a load cur-
rent change of 1mA to 300mA produces a 1.05mV drop
at the LFB input for the VLDO and a load current change
of 1mA to 30mA produces a 0.1mV drop at the LFB input
for the linear regulator. To calculate the change referred
to the output simply multiply by the gain of the feedback
network (i.e., 1 + R2/R1). For example, to program the
output for 1.2V choose R2/R1 = 2. In this example, an
output current change of 1mA to 300mA produces 1.05mV
" (1 + 2) = 3.15mV drop at the output.
Since the LFB pin is relatively high impedance (depending
on the resistor divider used), stray capacitance at this pin
should be minimized (<10pF) to prevent phase shift in the
error amplifier loop. Additionally, special attention should
be given to any stray capacitances that can couple external
signals onto the LFB pin producing undesirable output
ripple. For optimum performance connect the LFB pin to
R1 and R2 with a short PCB trace and minimize all other
stray capacitance to the LFB pin.
Output Capacitance and Transient Response
The LTC3541 is designed to be stable with a wide range of
ceramic output capacitors. The ESR of the output capaci-
tor affects stability, most notably with small capacitors.
A minimum output capacitor of 2.2礔 with an ESR of
0.05?or less is recommended to ensure stability. The
LTC3541 VLDO is a micropower device and output tran-
sient response will be a function of output capacitance.
Larger values of output capacitance decrease the peak
deviations and provide improved transient response for
larger load current changes. Note that bypass capacitors
used to decouple individual components powered by the
LTC3541 will increase the effective output capacitor value.
High ESR tantalum and electrolytic capacitors may be used,
but a low ESR ceramic capacitor must be in parallel at the
output. There is no minimum ESR or maximum capacitor
size requirement.
Figure 7. Programming the LTC3541
( )
LV
OUT
R1
R2
LTc3541
c
OUT
R2
R1
V
OUT
= 0.4V 1 +
LFB
GND
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