LTC3788 Datasheet, PDF (21/32 Pages) Linear Technology – 2-Phase, Dual Output Synchronous Boost Controller

LTC3788
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:
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.
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
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_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
V_OUT
INTV_CC Regulators
The LTC3788 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’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 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 V_IN • 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, the LTC3788
INTV_CC current is limited to less than 40mA from a 40V
supply when not using the EXTV_CC supply:
T_J = 70°C + (40mA)(40V)(34°C/W) = 125°C
LTC3788
R_B
VFB
R_A
3788 F04
Figure 4. Setting Output Voltage
LTC3788
SS
C_SS
SGND
3788 F05
Figure 5. Using the SS Pin to Program Soft-Start
3788f
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