LTC3124_15 Datasheet, PDF (18/28 Pages) Linear Technology – 15V, 5A 2-Phase Synchronous Step-Up DC/DC Converter with Output Disconnect

LTC3124
APPLICATIONS INFORMATION
For applications requiring a very low profile and very large
capacitance, the GS, GS2 and GW series from Cap-XX,
the BestCap series from AVX and PowerStor KR series
capacitors from Cooper all offer very high capacitance
and low ESR in various low profile packages.
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 can-
not tolerate any spectral noise. For example, in products
incorporating RF communications, the 455kHz IF frequency
can be 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
frequency 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:
f
MAX
_ NOSKIP
<
≅
V_OUT – V_IN
V_OUT • t_ON(MIN)
Hz
where t_ON(MIN) = minimum on-time, which is typically
around 100ns.
Thermal Considerations
For the LTC3124 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 accom-
plished by taking advantage of the large thermal pad on
the underside 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 trip an internal thermal shutdown,
and all switching will stop until the junction temperature
drops ~7°C.
Compensating the Feedback Loop
The LTC3124 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 7 shows the key equivalent small
signal elements of a boost converter.
The DC small-signal loop gain of the system shown in
Figure 7 is given by the following equation:
G_BOOST
=
G_EA
• G_MP
• G_POWER
•
R2
R1+ R2
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.
G_EA = g_ma • R_O ≈ 1000V/V
(Not Adjustable; g_ma ≈ 100µS, R_O ≈ 10MΩ)
G_MP
=
2 • g_mp;
g_mp
=
∆I_L
∆V_C
≈
3.4S
(Not
Adjustable)
G_POWER
=
∆V_OUT
∆I_L
=
η• V_IN
2 •I_OUT
=
η• V_IN •R_L
2 • V_OUT
3124f
18
For more information www.linear.com/LTC3124