LTC3788-1 Datasheet, PDF (18/28 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788-1
APPLICATIONS INFORMATION
of phases and the maximum output current. Figure 3 il-
lustrates the normalized output capacitor ripple current
as a function of duty cycle in a 2-phase configuration. To
choose a ripple current rating for the output capacitor,
first establish the duty cycle range based on the output
voltage and range of input voltage. Referring to Figure 3,
choose the worst-case high normalized ripple current as
a percentage of the maximum load current.
3.25
3.00
2.75
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0
0.1
1-PHASE
2-PHASE
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
DUTY CYCLE OR (1-VIN / VOUT)
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Figure 3. Normalized Output Capacitor Ripple
Current (RMS) for a Boost Converter
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 (i.e., OS-CON and POSCAP).
Setting Output Voltage
The LTC3788-1 output voltages are each set by an exter-
nal feedback resistor divider carefully placed across the
output, as shown in Figure 4. The regulated output voltage
is determined by:
V_OUT
=
1.2V
⎛
⎝⎜ ¹⁺
R_B
R_A
⎞
⎠⎟
Great care should be taken to route the V_FB line away from
noise sources, such as the inductor or the SW line.
V_OUT
LTC3788-1
R_B
VFB
R_A
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Figure 4. Setting Output Voltage
Soft-Start (SS Pins)
The start-up of each V_OUT is controlled by the voltage on
the respective SS pins. When the voltage on the SS pin
is less than the internal 1.2V reference, the LTC3788-1
regulates the VFB pin voltage to the voltage on the SS
pin instead of 1.2V.
LTC3788-1
SS
C_SS
SGND
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Figure 5. Using the SS Pin to Program Soft-Start
Soft-start is enabled by simply connecting a capacitor from
the SS pin to ground, as shown in Figure 5. An internal
10μA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3788-1 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:
1.2V
t_SS = C_SS • _10µA
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