4
DEMO MANUAL DC290
NO-DESIGN SWITCHER
OPERATIO
U
INTRODUCTION
The circuit in Figure 1 highlights the capabilities of the
LTC1877 and the LTC1878. The LTC1877 and the LTC1878
are high efficiency monolithic synchronous step-down
regulators using a fixed-frequency architecture.
This demo board is set up for a variety of output voltages.
Output voltages including 1.5V, 2.5V and 3.3V or user
programmable voltages can be obtained by selecting the
appropriate jumper position. For other output voltages,
select the “OPEN” position and add the appropriate resis-
tor value in the space provided. The output voltage must
never exceed 3.3V because the output capacitor may be
damaged. The input supply can range from 2.65V to 10V
for the LTC1877 and 2.65V to 6V for the LTC1878.
The operating frequency of this demo circuit is 550kHz.
For other frequencies, JP1 must be removed and SYNC/
MODE (E7) synchronized with an external clock. Burst
Mode operation is automatically disabled when SYNC/
MODE is externally driven. Grounding SYNC/MODE also
disables Burst Mode operation, potentially reducing noise
and RF interference.
This demonstration board is intended for the evaluation of
the LTC1877 and the LTC1878 switching regulator ICs and
was not designed for any other purpose.
Main Control Loop (Refer to Functional Diagram)
The LTC1877 and the LTC1878 use a constant frequency,
current mode step-down architecture. Their main and
synchronous switches, consisting of a top (main)
P-channel and a bottom (synchronous) N-channel power
MOSFET, are internal. During normal operation, the inter-
nal 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
resets the RS latch is
controlled by the voltage on the I
TH
pin, which is the output
of error amplifier EA. The V
FB
pin allows EA to receive an
output feedback voltage from an external resistive divider.
When the load current increases, it causes a slight de-
crease in the feedback voltage relative to the 0.8V refer-
ence, which, in turn, causes the I
TH
voltage to increase
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, as indicated by the current reversal comparator,
I
RCMP
, or until the beginning of the next clock cycle.
Comparator OVDET guards against transient overshoots
as well as other more serious conditions that may cause
an overvoltage condition on the output (> 6.25%). When
this condition is sensed, both MOSFETs are turned off until
the fault is removed.
Burst Mode Operation
The LTC1877 and the LTC1878 are capable of Burst Mode
operation, in which the internal power MOSFETs operate
intermittently based on load demand. To enable Burst
Mode operation, simply position jumper JP1 in the lower
position to connect SYNC/MODE to V
IN
. To disable Burst
Mode operation and enable PWM pulse skipping mode,
position JP1 in the upper position to connect SYNC/MODE
to GND (see Figure 5). In this mode, the efficiency is lower
at light loads, but becomes comparable to Burst Mode
operation when the output load exceeds 50mA. The
advantage of pulse skipping mode is lower output ripple
and less interference to audio circuitry.
When the converter is in Burst Mode operation, the peak
current of the inductor is set to approximately 250mA,
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, causing the internal sleep line to
go high and turn off both power MOSFETs. The I
TH
pin is
then disconnected from the output of the EA amplifier and
parked a diode 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 10
µ
A. 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.