8
LTC1875
1875f
OPERATIO
U
Main Control Loop
The LTC1875 uses a constant frequency, current mode
step-down architecture. Both the top MOSFET and syn-
chronous bottom MOSFET switches are internal. During
normal operation, the internal top power MOSFET is
turned on each cycle when the oscillator sets the RS latch,
and turned off when the current comparator, I
COMP
, resets
the RS latch. The peak inductor current at which I
COMP
turns the top MOSFET off is controlled by the voltage on
the I
TH
pin, which is the output of error amplifier EA. When
the load current increases, it causes a slight decrease in
the feedback voltage, V
FB
, relative to the 0.8V internal
reference, which, in turn, causes the I
TH
voltage to in-
crease until the average inductor current matches the new
load current. While the top MOSFET is off, the bottom
MOSFET is turned on until either the inductor current
starts to reverse direction or the next clock cycle begins.
Comparator OVDET guards against transient overshoots
>7.5% by turning the main switch off and keeping it off
until the fault is removed.
Burst Mode Operation
The LTC1875 is capable of Burst Mode operation in which
the internal power MOSFETs operate intermittently based
on load demand. To enable Burst Mode operation, simply
tie the SYNC/MODE pin to SV
IN
or connect it to a logic high
(V
SYNC/MODE
> 1.5V). To disable Burst Mode operation
and enable PWM pulse skipping mode, connect the SYNC/
MODE pin to SGND. In this mode, the efficiency is lower at
light loads but becomes comparable to Burst Mode opera-
tion when the output load exceeds 100mA. The advantage
of pulse skipping mode is lower output ripple.
When the converter is in Burst Mode operation, the peak
current of the inductor is set to approximately 400mA,
even though the voltage at the I
TH
pin indicates a lower
value. The voltage at the I
TH
pin drops when the inductor’s
average current is greater than the load requirement. As
the I
TH
voltage drops below approximately 0.45V, the
BURST comparator trips, turning off both power MOSFETs.
The I
TH
pin is then disconnected from the output of the EA
amplifier and held 0.65V above ground.
In sleep mode, both power MOSFETs are held off and the
internal circuitry is partially turned off, reducing the quies-
cent current to 15µA. The load current is now being
supplied from the output capacitor. When the output
voltage drops, the I
TH
pin reconnects to the output of the
EA amplifier and the top MOSFET is again turned on and
this process repeats.
Soft-Start/Run Function
The RUN/SS pin provides a soft-start function and a
means to shut down the LTC1875. Soft-start reduces the
input current surge by gradually increasing the regulator’s
maximum output current. This pin can also be used for
power supply sequencing.
Pulling the RUN/SS pin below 0.7V shuts down the
LTC1875, which then draws <1µA current from the sup-
ply. This pin can be driven directly from logic circuits as
shown in Figure 1. It is recommended that this pin is driven
to V
IN
during normal operation. Note that there is no
current flowing out of this pin. Soft-start action is accom-
plished by connecting an external RC network to the RUN/
SS pin as shown in Figure 1. The LTC1875 actively pulls
the RUN/SS pin to ground under low input supply voltage
conditions.
(Refer to Block Diagram)
3.3V OR 5V
V
IN
RUN/SS
D1*
0.32V
R
SS
C
SS
*ZETEX BAT54
1875 F01
Figure 1. RUN/SS Pin Interfacing
