LTC3788-1 Linear Technology, LTC3788-1 Datasheet - Page 18

• Manufacturer Part Number: LTC3788-1
• Description: Dual Output Synchronous Boost Controller
• Manufacturer: Linear Technology

www.DataSheet4U.com
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 confi guration. To
choose a ripple current rating for the output capacitor,
fi rst 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.
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 coeffi cient.
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:
18
V
OUT
=
Figure 3. Normalized Output Capacitor Ripple
Current (RMS) for a Boost Converter
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
1 2
0
.
0.1
V
0.2
⎛
⎜
⎝
1
DUTY CYCLE OR (1-V
+
0.3
R
R
B
A
0.4
⎞
⎟
⎠
1-PHASE
0.5
0.6
IN
/V
2-PHASE
0.7
OUT
)
0.8
37881 F03
0.9
Great care should be taken to route the V
noise sources, such as the inductor or the SW line.
Soft-Start (SS Pins)
The start-up of each V
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.
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
voltage on the SS pin, allowing V
from V
time will be approximately:
t
SS
IN
Figure 5. Using the SS Pin to Program Soft-Start
=
C
to its fi nal regulated value. The total soft-start
SS
Figure 4. Setting Output Voltage
•
10
. 1 2
µA
LTC3788-1
C
V
SS
OUT
VFB
is controlled by the voltage on
SS
SGND
LTC3788-1
V
37881 F05
OUT
R
R
OUT
OUT
37881 F04
B
A
) according to the
to rise smoothly
FB
line away from
37881f