Datasheet

LTC3833
20
3833f
APPLICATIONS INFORMATION
V
IN
Undervoltage Lockout (UVLO)
The LTC3833 has two functions that help protect the con-
troller in case of input undervoltage conditions. A precision
UVLO comparator constantly monitors the INTV
CC
voltage
to ensure that an adequate gate-drive voltage is present.
The comparator enables UVLO and locks out the switch-
ing action until INTV
CC
rises above 4.2V. Once UVLO is
released, the comparator does not retrigger UVLO until
INTV
CC
falls below 3.65V. This hysteresis prevents oscil-
lations when there are disturbances on INTV
CC
.
Another way to detect an undervoltage condition is to
monitor the V
IN
supply. Because the RUN pin has a precision
turn-on voltage of 1.2V, one can use a resistor divider from
V
IN
to turn on the IC when V
IN
is high enough. The RUN pin
has bias currents that depend on the RUN voltage as well
as V
IN
voltage. These bias currents should be taken into ac-
count when designing the voltage divider and UVLO circuit
to prevent faulty conditions. Generally for RUN < 3V a bias
current of 1.3μA flows out of the RUN pin, and for RUN >
3V, correspondingly increasing current flows into the pin,
reaching a maximum of about 35μA for RUN = 6V.
Soft-Start and Tracking
The LTC3833 has the ability to either soft-start by itself
with a capacitor or track the output of an external supply.
Soft-start or tracking features are achieved not by limiting
the maximum output current of the switching regulator
but by controlling the regulator’s output voltage according
to the ramp rate on the TRACK/SS pin.
When configured to soft-start by itself, a capacitor should
be connected to the TRACK/SS pin. TRACK/SS is pulled
low until the RUN pin voltage exceeds 1.2V and UVLO is
released, at which point an internal current of 1μA charges
the soft-start capacitor, C
SS
, connected to TRACK/SS.
Current foldback is disabled during this phase to ensure
smooth soft-start or tracking. The soft-start or tracking
range is defined to be the voltage range from 0V to 0.6V
on the TRACK/SS pin. The total soft-start time can be
calculated as:
t
SOFTSTART
= 0.6V •
C
SS
1µA
When the LTC3833 is configured to track another supply,
a voltage divider can be used from the tracking supply to
the TRACK/SS pin to scale the ramp rate appropriately.
Two common implementations of tracking as shown in
Figure 6a are coincident and ratiometric. For coincident
tracking, make the divider ratio from the external supply
the same as the divider ratio for the differential feedback
voltage. Ratiometric tracking could be achieved by using
a different ratio than the differential feedback (Figure 6b).
Note that the small soft-start capacitor charging current is
always flowing, producing a small offset error. To minimize
this error, select the tracking resistive divider values to be
small enough to make this offset error negligible.
Phase and Frequency Synchronization
For applications that require better control of EMI and
switching noise or have special synchronization needs, the
LTC3833 can phase and frequency synchronize the turn-on
of the top MOSFET to an external clock signal applied to
the MODE/PLLIN pin. The applied clock signal needs to
be within ±30% of the RT pin programmed free-running
frequency to assure proper frequency and phase lock.
The clock signal levels should generally comply to V
IH
>
2V and V
IL
< 0.5V. The MODE/PLLIN pin has an internal
600k pull-down resistor to ensure pulse-skipping mode
if the pin is left floating.
The LTC3833 uses the voltages on V
IN
and V
OUT
pins as
well as the RT programmed frequency to determine the
steady-state on-time as follows:
t
ON
≈
V
OUT
V
IN
• f
An internal PLL system adjusts this on-time dynamically
in order to maintain phase and frequency lock with the
external clock. The LTC3833 will maintain phase and fre-
quency lock under steady-state conditions for V
IN
, V
OUT
and load current.
As shown in the previous equation, the top MOSFET on-
time is a function of the switching regulator’s output. This
output is measured by the V
OUT
pin and is used to calculate
the required on-time. Therefore, simply connecting V
OUT
to the regulator’s local output point is preferable for most
applications. However, there could be applications where