LTC3802 Datasheet, PDF (16/28 Pages) Linear Technology – Dual 550kHz Synchronous 2-Phase DC/DC Controller with Programmable Up/Down Tracking

LTC3802
APPLICATIO S I FOR ATIO
internal 7µA current source pull-up is connected to the
RUN/SS pin, allowing a soft-start ramp to be generated
with a single external capacitor to ground. The 7µA current
source is active even when the LTC3802 is shut down,
ensuring the device will start when the external pull-down
at RUN/SS is released. Under shutdown conditions, the
LTC3802 goes into a micropower sleep mode, and the
quiescent current drops to 100µA.
The RUN/SS pin shuts down the LTC3802 when it falls
below 0.8V (Figure 2). Between about 0.8V and 2V, the
LTC3802 wakes up and the duty cycle is kept to a miminum.
As the potential at RUN/SS goes higher, the duty cycle
increases linearly between 2V and 3.2V, reaching its final
value of 89% when RUN/SS exceeds 3.2V. Prior to this
point, the feedback amplifier will assume control of the
loop and the output will come into regulation. Note that the
RUN/SS linear range varies with the potential at V_INFF; for
5V input voltage, the RUN/SS active range reduces to
2V-2.25V.
The value of the soft-start capacitor, C_SS, may depend on
the input and output voltages, inductor value, output
capacitance and load current. The inductor’s start-up
current (from V_OUT = 0V), can be much higher than its
steady-state current. The difference depends on the input
power supply slew rate, the input and output voltages, the
LTC3802 soft-start slew rate, and the inductor and output
capacitor values.
For a given application, the known input and output
requirements determine the output inductor and capacitor
V_OUT
0V
NORMAL OPERATION
START-UP
CURRENT LIMIT
5V
RUN/SS
COMP CONTROLS DUTY CYCLE
3.2V
RUN/SS CONTROLS DUTY CYCLE
2V
MINIMUM DUTY CYCLE
0.8V
0V
POWER DOWN MODE
LTC3802 ENABLE
3802 F02
Figure 2. Soft-Start Operation in Start-Up and Current Limit
16
values. These values establish the transient load recovery
time. In general, a low value inductor combined with a high
value capacitor yields a short transient load recovery time
at the expense of higher inductor ripple and start-up
current. These components, together with a small soft-
start capacitor, can also cause high inrash current. This
triggers the LTC3802 current limit comparator and forces
the LTC3802 to repeat the soft-start cycle, never allowing
the supply to start.
Start-up problems can also occur when a small soft-start
capacitor is used with a small output inductor and
capacitor. High input voltages generate high inrash cur-
rents, charging the output capacitor quickly and causing
the output to overshoot. The LTC3802 OVP comparator
turns off the top MOSFET once the output is 5% higher
than its nominal value. However, the residual energy in
the inductor will continue to charge the output capacitor,
forcing the output voltage to increase further until the
inductor energy is depleted. This overshoot at the output
causes the feedback loop to operate nonlinearly; the
output tends to ring for several cycles until the loop
mechanism is restored.
Therefore, select C_SS with start-up in mind. Choosing C_SS
to ensure that there is no output overshoot and the inrush
current is not able to trigger the current comparator. A
minimum recommended soft-start capacitor of
C_SS = 0.1µF will be sufficient for most applications.
Undervoltage Lockout
The LTC3802 is designed for wide V_IN operation. The
internal UVLO circuit monitors the V_CC and V_INFF potential
and starts operation as long as they are above their 2.5V
UVLO thresholds. For high V_IN supply operation, the low
UVLO threshold should not cause any problem under
typical application conditions. Upon power-up, once the
V_IN potential is higher than the UVLO threshold, the
LTC3802 releases the RUN/SS node and allows the start-
up current to charge the soft-start capacitor. The time
interval for the RUN/SS potential to ramp from 0.8V to 2V
allows the V_IN supply to slew to its steady-state potential.
A 0.1µF soft-start capacitor creates a 17ms time delay
before the driver starts switching. Most power supplies
have a start-up time well within this time interval. For some
3802f