LTC3784_15 Datasheet, PDF (22/38 Pages) Linear Technology – 60V PolyPhase Synchronous Boost Controller

LTC3784
Applications Information
LTC3784
VFB
V_OUT
R_B
R_A
3784 F05
Figure 5. Setting Output Voltage
LTC3784
SS
C_SS
SGND
3784 F06
Figure 6. Using the SS Pin to Program Soft-Start
INTV_CC Regulators
The LTC3784 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
LTC3784’s internal circuitry. The VBIAS LDO and the
EXTV_CC LDO regulate INTV_CC to 5.4V. Each of these can
supply at least 40mA 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 LTC3784 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 2 of the
Electrical Characteristics. For example, at 70°C ambient
temperature, the LTC3784 INTV_CC current is limited to less
than 21mA in the QFN package from a 60V VBIAS supply
when not using the EXTV_CC supply:
T_J = 70°C + (21mA)(60V)(43°C/W) = 125°C
In an SSOP package, the INTV_CC current is limited to
less than 11mA from a 60V supply when not using the
EXTV_CC supply:
T_J = 70°C + (11mA)(60V)(80°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
EXTV_CC remains above 4.55V. The EXTV_CC LDO attempts
to regulate the INTV_CC voltage to 5.4V, so while EXTV_CC
is less than 5.4V, the LDO is in dropout and the INTV_CC
voltage is approximately equal to EXTV_CC. When EXTV_CC
is greater than 5.4V, up to an absolute maximum of 14V,
INTV_CC is regulated to 5.4V.
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