LTC3216
8
3216fc
V
IN
, CPO Capacitor Selection
The style and value of capacitors used with the LTC3216
determine several important parameters such as regulator
control loop stability, output ripple, charge pump strength
and minimum start-up time.
To reduce noise and ripple, it is recommended that low
equivalent series resistance (ESR) ceramic capacitors be
used for both C
VIN
and C
CPO
. Tantalum and aluminum
capacitors are not recommended because of their high
ESR.
The value of C
CPO
directly controls the amount of output
ripple for a given load current. Increasing the size of
C
CPO
will reduce the output ripple at the expense of higher
start-up current. The peak-to-peak output ripple for 1.5x
mode is approximately given by the expression:
V
RIPPLE(P-P)
= I
OUT
/(3f
OSC
• C
CPO
) (3)
Where f
OSC
is the LTC3216’s oscillator frequency (typically
900kHz) and C
CPO
is the output storage capacitor.
Both the style and value of the output capacitor can sig-
nifi cantly affect the stability of the LTC3216. As shown in
the Block Diagram, the LTC3216 uses a control loop to
adjust the strength of the charge pump to match the cur-
rent required at the output. The error signal of this loop
is stored directly on the output charge storage capacitor.
The charge storage capacitor also serves as the dominant
pole for the control loop. To prevent ringing or instability,
it is important for the output capacitor to maintain at least
2.2µF of actual capacitance over all conditions.
Likewise, excessive ESR on the output capacitor will tend
to degrade the loop stability of the LTC3216. The closed
loop output resistance of the LTC3216 is designed to be
76m. For a 100mA load current change, the error signal
will change by about 7.6mV. If the output capacitor has
76m or more of ESR, the closed-loop frequency response
will cease to roll off in a simple one pole fashion and poor
load transient response of instability could result. Multilayer
ceramic chip capacitors typically have exceptional ESR
performance. MLCCs combined with a tight board layout
will yield very good stability. As the value of C
CPO
controls
the amount of output ripple, the value of C
VIN
controls the
amount of ripple present at the input pin (V
IN
). The input
current to the LTC3216 will be relatively constant while
the charge pump is on either the input charging phase or
the output charging phase but will drop to zero during the
clock nonoverlap times. Since the nonoverlap time is small
(~15ns), these missing “notches” will result in only a small
perturbation on the input power supply line. Note that a
higher ESR capacitor such as tantalum will have higher
input noise due to the input current change times the ESR.
Therefore, ceramic capacitors are again recommended for
their exceptional ESR performance. Input noise can be
further reduced by powering the LTC3216 through a very
small series inductor as shown in Figure 2. A 10nH inductor
will reject the fast current notches, thereby presenting a
nearly constant current load to the input power supply.
For economy, the 10nH inductor can be fabricated on the
PC board with about 1cm (0.4") of PC board trace.
OPERATION
whenever a dropout condition is detected at the I
LED
pin.
In the LOW current mode, the part will wait approximately
150ms after dropout is detected before switching to the
next mode. In the HIGH and LOW + HIGH current modes,
the part will wait approximately 2ms before switching to
the next mode. These delays allow the LED to warm up
and reduce its forward voltage which may remove the
dropout condition.
In order to reset the part back into 1x mode, the LTC3216
must be brought into shutdown (EN1 = EN2 = LOW). Im-
mediately after the part has been brought to shutdown,
it may be set to the desired output current level via the
EN1 and EN2 pins. An internal comparator will not allow
the main switches to connect V
IN
and CPO in 1x mode
until the voltage at the CPO pin has decayed to less than
or equal to the voltage at the V
IN
pin.
APPLICATIONS INFORMATION
