LTC3122
14
3122fa
For more information www.linear.com/LTC3122
a resistor across the top resistor of the feedback divider
(from V
OUT
to FB). This adds a phase-lead zero and pole
to the transfer function of the converter as calculated in
the Compensating the Feedback Loop section.
OPERATING FREQUENCY SELECTION
There are several considerations in selecting the operating
frequency of the converter. Typically the first consideration
is to stay clear of sensitive frequency bands, which cannot
tolerate any spectral noise. For example, in products incor
-
porating RF communications, the 455kHz IF frequency is
sensitive to any noise, therefore switching above 600kHz
is desired. Some communications have sensitivity to
1.1MHz and in that case a 1.5MHz switching converter
f
re
quency may be employed. A second consideration is the
physical size of the converter. As the operating frequency
is increased, the inductor and filter capacitors typically
can be reduced in value, leading to smaller sized external
components. The smaller solution size is typically traded
for efficiency, since the switching losses due to gate charge
increase with frequency.
Another consideration is whether the application can allow
pulse-skipping. When the boost converter pulse-skips, the
minimum on-time of the converter is unable to support
the duty cycle. This results in a low frequency component
to the output ripple. In many applications where physical
size is the main criterion, running the converter in this
mode is acceptable. In applications where it is preferred
not to enter this mode, the maximum operating frequency
is given by:
ƒ
MAX _NOSKIP
≤
OUT
−
IN
V
OUT
• t
ON(MIN)
Hz
where t
ON(MIN)
= minimum on-time = 100ns.
Thermal Considerations
For the LTC3122 to deliver its full power, it is imperative
that a good thermal path be provided to dissipate the heat
generated within the package. This can be accomplished
by taking advantage of the large thermal pad on the un
-
derside of the IC. It is recommended that multiple vias in
the printed circuit board be used to conduct heat away
from the IC and into a copper plane with as much area as
possible. If the junction temperature rises above ~170°C,
the part will go into thermal shutdown, and all switching
will stop until the temperature drops approximately 7°C.
Compensating the Feedback Loop
The LTC3122 uses current mode control, with internal
adaptive slope compensation. Current mode control elimi
-
nates the second order filter due to the inductor and output
capacitor exhibited in voltage mode control, and simplifies
the power loop to a single pole filter response. Because
of this fast current control loop, the power stage of the IC
combined with the external inductor can be modeled by a
transconductance amplifier g
mp
and a current controlled
current source. Figure 4 shows the key equivalent small
signal elements of a boost converter.
The DC small-signal loop gain of the system shown in
Figure 4 is given by the following equation:
G
BOOST
= G
EA
• G
MP
• G
POWER
•
where G
EA
is the DC gain of the error amplifier, G
MP
is
the modulator gain, and G
POWER
is the inductor current
to V
OUT
gain.
applicaTions inForMaTion
Figure 4. Boost Converter Equivalent Model
3122 F04
V
OUT
–
+
–
+
R
C
V
C
R
O
g
ma
g
mp
C
C
C
F
I
L
MODULATOR
ERROR
AMPLIFIER
1.202V
REFERENCE
R
PL
R1
FB
R2
R
ESR
R
L
C
PL
C
OUT
• I
L
η • V
IN
2 • V
OUT
C
C
: COMPENSATION CAPACITOR
C
OUT
: OUTPUT CAPACITOR
C
PL
: PHASE LEAD CAPACITOR
C
F
: HIGH FREQUENCY FILTER CAPACITOR
g
ma
: TRANSCONDUCTANCE AMPLIFIER INSIDE IC
g
mp
: POWER STAGE TRANSCONDUCTANCE AMPLIFIER
R
C
: COMPENSATION RESISTOR
R
L
: OUTPUT RESISTANCE DEFINED AS V
OUT
/I
LOADMAX
R
O
: OUTPUT RESISTANCE OF g
ma
R
PL
: PHASE LEAD RESISTOR
R1, R2: FEEDBACK RESISTOR DIVIDER NETWORK
R
ESR
: OUTPUT CAPACITOR ESR
η : CONVERTER EFFICIENCY (~90% AT HIGHER CURRENTS)
