Datasheet

LTC3787
21
3787fc
APPLICATIONS INFORMATION
Setting Output Voltage
The LTC3787 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 keep the VFB node as small as possible to avoid
noise pickup.
INTV
CC
Regulators
The LTC3787 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
LTC3787’s internal circuitry. The VBIAS LDO and the
EXTV
CC
LDO regulate INTV
CC
to 5.4V. Each of these can
supply at least 50mA and must be bypassed to ground with
a minimum of 4.7F ceramic capacitor. Good bypassing
is needed to supply the high transient currents required
by the MOSFET gate drivers and to prevent interaction
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 LTC3787 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
case, power dissipation for the IC is highest and is equal
to VBIAS • I
INTVCC
. The gate charge current is dependent
on operating frequency, as discussed in the Efficiency
Considerations section. The junction temperature can be
estimated by using the equations given in Note 3 of the
Electrical Characteristics. For example, at 70°C ambient
temperature, the LTC3787 INTV
CC
current is limited to less
than 32mA in the QFN package from a 40V VBIAS supply
when not using the EXTV
CC
supply:
T
J
= 70°C + (32mA)(40V)(43°C/W) = 125°C
In an SSOP package, the INTV
CC
current is limited to
less than 15mA from a 40V supply when not using the
EXTV
CC
supply:
T
J
= 70°C + (15mA)(40V)(90°C/W) = 125°C
To prevent the maximum junction temperature from being
exceeded, the input supply current must be checked while
operating in continuous conduction mode (PLLIN/MODE
= INTV
CC
) at maximum V
IN
.
When the voltage applied to EXTV
CC
rises above 4.8V, the
V
IN
LDO is turned off and the EXTV
CC
LDO is enabled. The
EXTV
CC
LDO remains on as long as the voltage applied to
Figure 6. Using the SS Pin to Program Soft-Start
LTC3787
SS
C
SS
SGND
3787 F06
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 LTC3787 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
10A current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3787 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 5. Setting Output Voltage
LTC3787
VFB
V
OUT
R
B
R
A
3787 F05