LTC3810-5 Datasheet, PDF (22/36 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3810-5
APPLICATIONS INFORMATION
The safe operating area (SOA) for the external NMOS
should be chosen so that capacitor charging does not
damage the NMOS. Excessive values of capacitor are
unnecessary and should be avoided. Typically values in
the 1μF to 10μF work well.
One more design requirement for this mode is the minimum
soft-start capacitor value. The fault timeout is enabled
when SS/TRACK voltage is greater than 4V. This gives the
power supply time to bring the output up before it starts
the timeout sequence. To prevent timeout sequence from
starting prematurely during start-up, a minimum C_SS value
is necessary to ensure that V_SS/TRACK < 4V until V_EXTVCC
> 4.7V. To ensure this, choose:
C_SS > C_OUT • (2.3 • 10^–6)/I_OUT(MAX)
Mode 2 should be used if V_OUT is outside of the 4.7V <
EXTV_CC < 15V operating range and the extra complexity
of a charge pump or extra inductor winding is not wanted
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_TH)/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 1Meg resistor
from SS/TRACK pin 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 and C_DRVCC through R_PULLUP (t_DELAY = 0.77
• R_PULLUP • C_DRVCC) and usually requires larger INTV_CC/
DRV_CC capacitor values to hold up the supply voltage dur-
ing start-up. Once the INTV_CC/DRV_CC voltage reaches the
trickle charge UV threshold of 9V, the drivers will turn on
and start discharging C_INTVCC/C_DRVCC at a rate determined
22
by the driver current I_G. In order to ensure proper start-
up, C_INTVCC/C_DRVCC must be chosen large enough so that
the EXTV_CC voltage reaches the switchover threshold of
4.7V before C_INTVCC/C_DRVCC discharges below the falling
UV threshold of 4V. This is ensured if:
C_INTVCC + C_DRVCC >


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/
DRV_CC Pins
If an external supply is available between 4.5V and 14V,
the supply can be connected directly to the INTV_CC/DRV_CC
pins. In this mode, INTV_CC, EXTV_CC and NDRV must be
shorted together.
INTV_CC/DRV_CC Supply and the EXTV_CC Connection
The LTC3810-5 contains an internal low dropout regulator
to produce the 5.5V INTV_CC/DRV_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
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