ltc3787gn Linear Technology Corporation, ltc3787gn Datasheet - Page 21

• Manufacturer Part Number: ltc3787gn
• Description: Ltc3787 - Polyphase Synchronous Boost Controller
• Manufacturer: Linear Technology Corporation

Soft-Start (SS Pin)
The start-up of V
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
10μA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3787 will
regulate the VFB pin (and hence, V
voltage on the SS pin, allowing V
from V
time will be approximately:
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:
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.
t
V
SS
OUT
= C
IN
Figure 6. Using the SS Pin to Program Soft-Start
= 1.2V 1+
to its final regulated value. The total soft-start
SS
•
Figure 5. Setting Output Voltage
10µA
1.2V

 
OUT
LTC3787
R
R
C
is controlled by the voltage on the
SS
B
A

 
VFB
SS
SGND
LTC3787
V
OUT
3787 F06
R
R
OUT
OUT
3787 F05
B
A
) according to the
to rise smoothly
INTV
The LTC3787 features two separate internal P-channel
low dropout linear regulators (LDO) that supply power at
the INTV
EXTV
pin. INTV
LTC3787’s internal circuitry. The VBIAS LDO and the
EXTV
supply at least 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 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
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 VBIAS • I
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
than 32mA in the QFN package from a 40V VBIAS supply
when not using the EXTV
In an SSOP package, the INTV
less than 15mA from a 40V supply when not using the
EXTV
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
When the voltage applied to EXTV
V
EXTV
IN
T
T
LDO is turned off and the EXTV
J
J
CC
CC
CC
CC
CC
= 70°C + (32mA)(40V)(43°C/W) = 125°C
= 70°C + (15mA)(40V)(90°C/W) = 125°C
CC
Regulators
supply:
LDO remains on as long as the voltage applied to
pin depending on the connection of the EXTV
LDO regulate INTV
CC
) at maximum V
CC
INTVCC
pin from either the VBIAS supply pin or the
powers the gate drivers and much of the
CC
. The gate charge current is dependent
LDO. When the voltage on the EXTV
CC
current, which is dominated by the
CC
IN
.
CC
supply:
to 5.4V. Each of these can
CC
CC
current is limited to less
CC
CC
current is limited to
rises above 4.8V, the
LDO is enabled. The
LTC3787
21
3787fa
CC
CC