Linear Technology LTC1877 Demo Manual page 6

DEMO MANUAL DC290
NO-DESIGN SWITCHER
U
OPERATIO
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle until it reaches 100% duty cycle. The output voltage
will then be determined by the input voltage minus the
voltage drop across the P-channel MOSFET and the
inductor.
Low Supply Operation
The LTC1877 and the LTC1878 can function on an input
supply voltage as low as 2.65V. The maximum allowable
output current is reduced at this low voltage because the
R of the P-channel switch increases. Therefore, the
DS(ON)
user should calculate the power dissipation when the
LTC1877 or the LTC1878 is used at 100% duty cycle with
low V . See the LTC1877 or the LTC1878 data sheet for
IN
additional information.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at high duty cycles. It is accomplished by internally
adding a compensating ramp to the inductor current
signal at duty cycles in excess of 40%. As a result, the
maximum inductor peak current is reduced for duty
cycles >40%. See the inductor peak current as a function
of duty cycle graph in Figure 4.
+
+
V
IN
6
High Efficiency Monolithic Synchronous
RUN
E1
UPPER POSITION (U)
I
IN
LOWER POSITION (L)
A
E2
V
IN
+
V
GND
E3
Figure 5. Proper Measurement Setup
1100
1000
900
800
700
600
0
Figure 4. Maximum Inductor Peak Current vs Duty Cycle
How To Measure Voltage Regulation
When trying to measure voltage regulation, remember
that all measurements must be taken at the point of
regulation. This point is where the LTC1877's and
LTC1878's control loop looks for the information to keep
the output voltage constant. In this demonstration board,
this point occurs between V
Measurements should be taken at these points and not at
the end of test leads at the load. Refer to Figure 5 for the
proper monitoring equipment configuration.
This applies to line regulation (input-to-output voltage
regulation) as well as load regulation tests. In doing the
line regulation tests, always look at the input voltage
across the input terminals, V
LTC1877/LTC1878
Step-Down Regulator
SYNC/MODE
E7
JP2 JP1
V
OUT
E6
GND
E5
JP3 JP3 JP3
GND
E4
DEMO CIRCUIT DC290A
(408) 432-1900
LTC1877 @ V = 5V
IN
LTC1878 @ V = 3.3V
IN
20 40 60 80 100
DUTY CYCLE (%)
dc290A F04
(E6) and GND (E4).
OUT
(E2) and GND (E3) .
IN
I
OUT
+
A
+
LOAD
V V
OUT