LTC3812-5_15 Datasheet, PDF (21/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
APPLICATIONS INFORMATION
to boost this voltage above 4.7V. In this mode, EXTV_CC is
grounded and the NMOS is chosen to handle the worst-
case power dissipation:
P_MOSFET = (V_IN(MAX))[(f)(Q_G(TOP) + Q_G(BOTTOM) + 3mA]
To operate properly, the fault timeout operation must be
disabled by choosing
R_NDRV > (V_IN(MAX) – 5.5V – V_T)/270μA
If the required R_NDRV value results in an unacceptable
value for V_IN(MIN) (see Equation 1), fault timeout operation
can also be disabled by connecting a 500k to 1M resistor
from RUN/SS to INTV_CC.
Using Trickle Charge Mode
Trickle charge mode is selected by shorting NDRV and
INTV_CC and connecting EXTV_CC to V_OUT. Trickle charge mode
has the advantage of not requiring an external MOSFET but
takes longer to start up due to slow charge up of C_INTVCC
through R_PULLUP (t_DELAY = 0.77 • R_PULLUP • C_INTVCC) and
usually requires a larger INTV_CC capacitor value to hold
up the supply voltage during start-up. Once the INTV_CC
voltage reaches the trickle charge UV threshold of 9V, the
drivers will turn on and start discharging C_INTVCC at a rate
determined by the driver current I_G. In order to ensure
proper start-up, C_INTVCC must be chosen large enough so
that the EXTV_CC voltage reaches the switchover threshold
of 4.7V before C_INTVCC discharges below the falling UV
threshold of 4V. This is ensured if:
C_INTVCC
> I_G


• _Larger of

C_OUT
I_MAX
or
5.5 • 10⁵ • C_SS
V_OUT(REG)



where I_G is the gate drive current = (f)(Q_G(TOP) + Q_G(BOTTOM))
and I_MAX is the maximum inductor current selected by
V_RNG.
For R_PULLUP, the value should fall in the following range
to ensure proper start-up:
Min R_PULLUP > (V_IN(MAX) – 14V)/I_CCSR
Max R_PULLUP < (V_IN(MIN) – 9V)/I_Q,SHUTDOWN
Using an External Supply Connected to the INTV_CC
If an external supply is available between 4.2V and 14V,
the supply can be connected directly to the INTV_CC pins.
In this mode, INTV_CC, EXTV_CC and NDRV must be shorted
together.
INTV_CC Supply and the EXTV_CC Connection
The LTC3812-5 contains an internal low dropout regula-
tor to produce the 5.5V INTV_CC supply from the EXTV_CC
pin voltage. This regulator turns on when the EXTV_CC pin
is above 4.7V and remains on until EXTV_CC drops below
4.45V. This allows the IC/MOSFET power to be derived
from the output or an output derived boost network during
normal operation and from the external NMOS from V_IN
during start-up or short-circuit. Using the EXTV_CC pin in
this way results in significant efficiency gains compared
to what would be possible when deriving this power
continuously from the typically much higher V_IN voltage.
The EXTV_CC connection also allows the power supply to
be configured in trickle charge mode in which it starts up
with a high-valued “bleed” resistor connected from V_IN
to INTV_CC to charge up the INTV_CC capacitor. As soon as
the output rises above 4.7V the internal EXTV_CC regulator
takes over before the INTV_CC capacitor discharges below
the UV threshold. When the EXTV_CC regulator is active,
the EXTV_CC pin can supply up to 50mA RMS. Do not ap-
ply more than 15V to the EXTV_CC pin. The following list
summarizes the possible connections for EXTV_CC:
1. EXTV_CC grounded. This connection will require INTV_CC
to be powered continuously from an external NMOS
from V_IN resulting in an efficiency penalty as high as
10% at high input voltages.
2. EXTV_CC connected directly to V_OUT. This is the normal
connection for 4.7V < V_OUT < 15V and provides the
highest efficiency. The power supply will start up using
an external NMOS or a bleed resistor until the output
supply is available.
3. EXTV_CC connected to an output-derived boost network.
If V_OUT < 4.7V. The low voltage output can be boosted
using a charge pump or flyback winding to greater than
4.7V.
4. EXTV_CC connected to INTV_CC. This is the required
connection for EXTV_CC if INTV_CC is connected to an
external supply where the external supply is 4.2V <
V_EXT < 14V.
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