LTC3784_15 Datasheet, PDF (20/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Applications Information
Multiple capacitors placed in parallel may be needed to
meet the ESR and RMS current handling requirements.
Dry tantalum, special polymer, aluminum electrolytic and
ceramic capacitors are all available in surface mount
packages. Ceramic capacitors have excellent low ESR
characteristics but can have a high voltage coefficient.
Capacitors are now available with low ESR and high ripple
current ratings (e.g., OS-CON and POSCAP).
PolyPhase Operation
For output loads that demand high current, multiple
LTC3784s can be cascaded to run out-of-phase to provide
more output current and at the same time to reduce input
and output voltage ripple. The PLLIN/MODE pin allows the
LTC3784 to synchronize to the CLKOUT signal of another
LTC3784. The CLKOUT signal can be connected to the
PLLIN/MODE pin of the following LTC3784 stage to line
up both the frequency and the phase of the entire system.
Tying the PHASMD pin to INTV_CC, SGND or floating
generates a phase difference (between PLLIN/MODE
and CLKOUT) of 240°, 60° or 90°, respectively, and a
phase difference (between CH1 and CH2) of 120°, 180°
or 180°. Figure 4 shows the connections necessary for
3-, 4-, 6- or 12-phase operation. A total of 12 phases can
be cascaded to run simultaneously out-of-phase with
respect to each other.
Setting Output Voltage
The LTC3784 output voltage is set by an external feedback
resistor divider carefully placed across the output, as shown
in Figure 5. The regulated output voltage is determined by:
V_OUT
=
1.2V
⎛
⎝⎜
1+
R_B
R_A
⎞
⎠⎟
Great care should be taken to route the VFB line away
from noise sources, such as the inductor or the SW line.
Also place the feedback resistor divider close to the VFB
pin and keep the VFB node as small as possible to avoid
noise pickup.
Soft-Start (SS Pin)
The start-up of V_OUT is controlled by the voltage on the
SS pin. When the voltage on the SS pin is less than the
internal 1.2V reference, the LTC3784 regulates the VFB
pin voltage to the voltage on the SS pin instead of 1.2V.
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 6. An internal
10μA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3784 will
regulate the VFB pin (and hence, V_OUT) according to the
voltage on the SS pin, allowing V_OUT to rise smoothly
from V_IN to its final regulated value. The total soft-start
time will be approximately:
t_SS
=
C_SS
•
1.2V
10µA
20
For more information www.linear.com/LTC3784
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