LTC3788 Linear Technology, LTC3788 Datasheet - Page 21

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

www.DataSheet4U.com
APPLICATIONS INFORMATION
Setting Output Voltage
The LTC3788 output voltages are each set by an external
feedback resistor divider carefully placed across the out-
put, as shown in Figure 4. The regulated output voltage
is determined by:
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
on the respective SS pins. When the voltage on the SS
pin is less than the internal 1.2V reference, the LTC3788
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 will
regulate the VFB pin (and hence, V
voltage on the SS pin, allowing V
from V
time will be approximately:
V
t
SS
OUT
IN
=
=
C
to its fi nal regulated value. The total soft-start
SS
1 2
.
Figure 4. Setting Output Voltage
•
V
10
. 1 2
⎛
⎝ ⎜
1
µA
LTC3788
+
V
R
R
OUT
B
A
VFB
⎞
⎠ ⎟
is controlled by the voltage
V
OUT
R
R
3788 F04
OUT
OUT
B
A
) according to the
FB
to rise smoothly
line away from
INTV
The LTC3788 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
EXTV
pin. INTV
LTC3788’s internal circuitry. The VBIAS LDO and the
EXTV
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 to be
exceeded. The INTV
gate charge current, may be supplied by either the VBIAS
LDO or the EXTV
pin is less than 4.8V, the VBIAS LDO is enabled. In this
case, power dissipation for the IC is highest and is equal
to V
on operating frequency, as discussed in the Effi ciency
Considerations section. The junction temperature can
be estimated by using the equations given in Note 3 of
the Electrical Characteristics. For example, the LTC3788
INTV
supply when not using the EXTV
T
J
IN
CC
CC
CC
CC
= 70°C + (40mA)(40V)(34°C/W) = 125°C
• I
Figure 5. Using the SS Pin to Program Soft-Start
Regulators
current is limited to less than 40mA from a 40V
pin depending on the connection of the EXTV
LDO regulate INTV
CC
CC
INTVCC
pin from either the VBIAS supply pin or the
powers the gate drivers and much of the
. The gate charge current is dependent
CC
LDO. When the voltage on the EXTV
CC
C
SS
current, which is dominated by the
SS
SGND
CC
LTC3788
to 5.4V. Each of these can
3788 F05
CC
supply:
LTC3788
21
3788f
CC
CC