Datasheet

LTC3788-1
18
37881fc
APPLICATIONS INFORMATION
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.
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:
t
SS
= C
SS
•
1.2V
10µA
Figure 3. Normalized Output Capacitor Ripple
Current (RMS) for a Boost Converter
Figure 4. Setting Output Voltage
Figure 5. Using the SS Pin to Program Soft-Start
0.1
I
ORIPPLE
/I
OUT
0.9
37881 F03
0.3
0.5
0.7
0.8
0.2
0.4
0.6
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
DUTY CYCLE OR (1-V
IN
/V
OUT
)
1-PHASE
2-PHASE
LTC3788-1
VFB
V
OUT
R
B
R
A
37881 F04
LTC3788-1
SS
C
SS
SGND
37881 F05
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.
INTV
CC
Regulators
The LTC3788-1 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
CC
pin from either the VBIAS supply pin or the
EXTV
CC
pin depending on the connection of the EXTV
CC
pin. INTV
CC
powers the gate drivers and much of the
LTC3788-1’s internal circuitry. The VBIAS LDO and the
EXTV
CC
LDO regulate INTV
CC
to 5.4V. Each of these can
supply a peak current of 50mA and must be bypassed to
ground with a minimum of 4.7µF ceramic capacitor. Good
bypassing is needed to supply the high transient currents
required by the MOSFET gate drivers and to prevent in-
teraction between the channels.
High input voltage applications in which large MOSFETs
are being driven at high frequencies may cause the maxi-
mum junction temperature rating for the LTC3788-1 to be
exceeded. The INTV
CC
current, which is dominated by the
gate charge current, may be supplied by either the VBIAS
LDO or the EXTV
CC
LDO. When the voltage on the EXTV
CC
pin is less than 4.8V, the VBIAS LDO is enabled. In this