LTC3769_15 Datasheet, PDF (19/32 Pages) Linear Technology – 60V Low IQ Synchronous Boost Controller

LTC3769
Applications Information
Setting Output Voltage
The LTC3769 output voltage is set by an external feedback
resistor divider carefully placed across the output, as shown
in Figure 3. 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 place the feedback resistor divider close to the VFB
pin and keep the VFB node as small as possible to avoid
noise pickup.
LTC3769
VFB
V_OUT
R_B
R_A
3769 F03
Figure 3. Setting Output Voltage
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 LTC3769 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 4. An internal
10μA current source charges the capacitor, providing a
linear ramping voltage at the SS pin. The LTC3769 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
LTC3769
SS
C_SS
GND
3769 F04
Figure 4. Using the SS Pin to Program Soft-Start
INTV_CC Regulators
The LTC3769 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
LTC3769’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 a 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 LTC3769 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 LTC3769 INTV_CC current is limited to less
than 19mA in the QFN package from a 60V VBIAS supply
when not using the EXTV_CC supply:
T_J = 70°C + (19mA)(60V)(47°C/W) = 125°C
For more information www.linear.com/LTC3769
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