LTC3424_15 Datasheet, PDF (9/12 Pages) Linear Technology – Low Output Voltage, 3MHz Micropower Synchronous Boost Converters

LTC3423/LTC3424
APPLICATIO S I FOR ATIO
Operating Frequency Selection
There are several considerations in selecting the operat-
ing frequency of the converter. The first is determining the
sensitive frequency bands that cannot tolerate any spec-
tral noise. For example, in products incorporating RF
communications, the 455kHz IF frequency is sensitive to
any noise, therefore switching above 600kHz is desired.
Some communications have sensitivity to 1.1MHz. In this
case, converter frequencies up to 3MHz may be em-
ployed.
The second consideration is the physical size of the
converter. As the operating frequency goes up, the induc-
tor and filter caps go down in value and size. The trade off
is in efficiency since the switching losses due to gate
charge are going up proportional with frequency.
Another operating frequency consideration is whether the
application can allow “pulse skipping.” In this mode, the
minimum on time of the converter cannot support the duty
cycle, so the converter ripple will go up and there will be
a low frequency component of the output ripple. In many
applications where physical size is the main criterion then
running the converter in this mode is acceptable. In
applications where it is preferred not to enter this mode,
then the maximum operating frequency is given by:
f_{MAX _NOSKIP}
=
V_OUT – V_IN
V_OUT • t_ON(MIN)
Hz
where t_ON(MIN) = minimum on time = 140ns
Reducing Output Capacitance with a Load Feed
Forward Signal
In many applications the output filter capacitance can be
reduced for the desired transient response by having the
device commanding the change in load current, (i.e.
system microcontroller), inform the power converter of
the changes as they occur. Specifically, a “load feed
forward” signal coupled into the V_C pin gives the inner
current loop a head start in providing the change in output
current. The transconductance of the LTC3423 converter
at the V_C pin with respect to the inductor current is typically
130mA/100mV, and the LTC3424 is typically 170mA/
100mV, so the amount of signal injected is proportional to
the anticipated change of inductor current with load. The
outer voltage loop performs the remainder of the correc-
tion, but because of the load feed forward signal, the range
over which it must slew is greatly reduced. This results in
an improved transient response. A logic level feed forward
signal, V_FF, is coupled through components C5 and R6.
The amount of feed forward signal is attenuated with
resistor R6 and is given by the following relationship:
R6
≈


V_FF •R5 • V_IN •1.5^
V_OUT • ∆I_OUT 
–
R5
where ∆I_OUT = load current change.
V_IN
LTC3423/LTC3424
V_DD
IN
6
V_DD
4
SW
10
SHDN
7
V_OUT
3
V_IN
8
FB
2
9
MODE/SYNC V_C
1
5
C3
R_t
GND
R5
LOAD FEED
FORWARD
SIGNAL
C5
R6 ^3.3nF
V_FF
Figure 2
V_OUT
3423/24 F02
Closing the Feedback Loop
The LTC3423/LTC3424 uses current mode control with
internal adaptive slope compensation. Current mode con-
trol eliminates the 2nd order filter due to the inductor and
output capacitor exhibited in voltage mode controllers,
and simplifies it to a single-pole filter response. The
product of the modulator control to output DC gain plus
the error amp open-loop gain equals the DC gain of the
system.
34234f
9