DEMO MANUAL DC290
NO-DESIGN SWITCHER
LTC1877/LTC1878
Monolithic Synchronous
Step-Down Regulator
voltage, by means of a jumper. The frequency is internally
set at 550kHz or can be synchronized with an external
clock. The internal switches allow up to 600mA of output
current in an MSO8 package, providing a space-efficient
solution for battery-powered applications. The DC supply
current is typically only 10µA at no load and less than 1µA
in shutdown. In switching-noise sensitive applications,
Burst Mode
TM
operation can be inhibited by grounding the
SYNC/MODE pin with a jumper or synchronizing it with an
external clock.
Gerber files for this circuit board are
available. Call the LTC factory.
DESCRIPTIO
Demonstration circuit DC290 is a constant-frequency
step-down converter using an LTC
®
1877 or an LTC1878
monolithic synchronous regulator. The LTC1878 has an
input voltage range of 2.65V < V
IN
< 6V and the LTC1877
has an input voltage range of 2.65V < V
IN
< 10V. The
LTC1878 is optimized for low voltage operation and is
ideally suited for single Li-Ion cell or 3-NiCd/NiMH cell
applications. The LTC1877, with its higher voltage capa-
bility, is ideally suited for two Li-Ion cells or 4- to 6-NiCd/
NiMH cell applications.
The exclusive use of low profile surface mount compo-
nents on this demo board results in a highly efficient
application in a small volume. The output voltage can be
selected from 1.5V, 2.5V, 3.3V or a user programmable
PERFORmANCE SU
SYMBOL PARAMETER
V
IN
V
OUT
Input Voltage Range
Output Voltage
TYPICAL PERFORMANCE CHARACTERISTICS AND BOARD PHOTO
EFFICIENCY CURVE
100
95
90
85
EFFICIENCY (%)
80
75
70
65
60
55
50
0.1
V
OUT
= 3.3V
L = 10µH
Burst Mode OPERATION
1.0
100
10
OUTPUT CURRENT (mA)
1000
290A • TA01
V
IN
= 3.6V
V
IN
= 10V
V
IN
= 7.2V
V
IN
= 5V
U
WW
U W
U
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
U W
ARY
CONDITIONS
LTC1877
LTC1878
V
IN
= 5V, SYNC/MODE = 0V, RUN = 5V, I
OUT
= 0mA
V
IN
= 5V, SYNC/MODE = 0V, RUN = 5V, I
OUT
= 0mA
V
IN
= 5V, SYNC/MODE = 0V, RUN = 5V, I
OUT
= 0mA
V
IN
= 5V, SYNC/MODE = 0V, RUN = 5V, I
OUT
= 0mA
JUMPER POSITION
All (Note 1)
All
VALUE
2.65V to 10V
2.65V to 6V
JP1 = U, JP2 = L, JP3 = “1.5V” 1.51V
±
0.05V
JP1 = U, JP2 = L, JP3 = “2.5V” 2.52V
±
0.08V
JP1 = U, JP2 = L, JP3 = “3.3V” 3.33V
±
0.10V
JP1 = U, JP2 = L, JP3 = “OPEN”
(Note 2)
DEMO BOARD
1
DEMO MANUAL DC290
NO-DESIGN SWITCHER
PERFOR A CE SU
SYMBOL PARAMETER
I
Q
Burst Mode Operation Enabled
Supply Current
Pulse Skipping Mode Supply Current
Shutdown Current
I
OUT
f
OSC
V
RIPPLE
V
OUT
V
SYNC
V
RUN
Maximum Output Current
Operating Frequency
Typical Output Ripple
Typical Load Regulation
Synchronize Threshold Voltage
Minimum Shutdown Threshold Voltage
NOTE 1:
JP1 and JP2 connect RUN (E1) and SYNC/MODE (E7) to either
GND (E3) or V
IN
(E2). When JP1 or JP2 is in the lower position (L), then
RUN or SYNC/MODE is connected to V
IN
. Alternatively, if JP1 or JP2 is in
the upper position (U), then RUN or SYNC/MODE is connected to GND. If
external sources drive RUN or SYNC/MODE, then JP1, JP2 or both must
be removed.
PACKAGE A D SCHE ATIC DIAGRA SM
TOP VIEW
JP2
JP1
E2
V
IN
≤
10V (LTC1877)
≤
6V (LTC1878)
C1
0.01µF R1
10k
E1
RUN
1
OPTIONAL
R2
C2
C
ITH
220pF
2
3
4
RUN
I
TH
V
FB
GND
PLL LPF
SYNC/MODE
LTC187X
V
IN
SW
8
7
6
5
L1
10µH
C
FW
20pF
+
R3
887k
C
OUT
47µF
6.3V
E6
V
OUT
C
OUT1
**
1µF
CER
E7
SYNC/MODE
R4
1M
E3
GND
1.5V
R5
412k
2.5V
R6
280k
3.3V
R7*
OPTIONAL
OPEN
E5
GND
E4
GND
JP3
* SPACE PROVIDED FOR AN OPTIONAL RESISTOR TO PROGRAM A CUSTOM OUTPUT VOLTAGE. THE OUTPUT VOLTAGE MUST NOT EXCEED 3.3V.
** C
OUT1
IS AN OPTIONAL CAPACITOR TO FILTER OUT VERY HIGH FREQUENCY SWITCHING NOISE.
*** C
IN
= 10µF FOR LTC1877 AND C
IN
= 22µF FOR LTC1878
Figure 1. LTC1877/LTC1878 Constant Frequency, High Efficiency Converter
2
W
WW
W
U W
ARY
CONDITIONS
V
IN
= 5V, SYNC/MODE = RUN = 5V, I
OUT
= 0mA
V
IN
= 5V, SYNC/MODE = 0V, RUN = 5V, I
OUT
= 0mA
V
IN
= 5V, RUN = 0V
V
IN
= 5V, V
OUT
= 2.5V (LTC1877)
V
IN
= 4V, V
OUT
= 2.5V (LTC1878)
Unsynchronized
Synchronized
I
OUT
= 600mA, V
IN
= 5V, V
OUT
= 2.5V
0mA < I
OUT
< 0.6A, V
IN
= 5V, SYNC/MODE = 0V
JUMPER POSITION
JP1 = JP2 = L, JP3 = “1.5V”
JP1 = U, JP2 = L
JP2 = U
JP2 = L, JP3 = “1.5V”
JP2 = L, JP3 = “1.5V”
JP2 = L
JP1 = OPEN, JP2 = L
JP2 = L, JP3 = “2.5V”
JP1 = U, JP2 = L
All
All
VALUE
10µA (Note 3)
230µA
<1µA
600mA
600mA
550kHz
400kHz to 700kHz
20mV
P-P
0.3%
1.2V
0.4V
NOTE 2:
Programmable via optional R7. V
OUT
= 0.8V(1+ 887kΩ/R7)
NOTE 3:
With V
IN
= 5V, the external feedback resistor contributes 0.24µA
with JP3 = “1.5V” selected, 0.95µA with JP3 = “2.5V” selected and 1.85µA
with JP3 = “3.3V” selected.
U
RUN
I
TH
V
FB
GND
1
2
3
4
8
7
6
5
PPL LPF
SYNC/MODE
V
IN
SW
C
IN
***
CER
MS8 PACKAGE
8-LEAD PLASTIC MSOP
LTC187XEMS8
DEMO MANUAL DC290
NO-DESIGN SWITCHER
PARTS LIST
REFERENCE
DESIGNATOR
C
FW
C
ITH
C1
C
IN
(LTC1877)
C
IN
(LTC1878)
C
OUT
C
OUT1
E1 to E7
JP1, JP2
JP3
JP1 to JP3
L1
R1
R3
R4
R5
R6
U1
C2
R2, R7
QUANTITY PART NUMBER
1
1
1
1
1
1
1
7
2
1
3
1
1
1
1
1
1
1
06035A200JAT
06035A221JAT
06035C103MAT
LMK325BJ106MN
JMK325BJ226MM
6TPA47M
LMK107F105ZA
2501-2
2802S-03-G1
2202S-08-G1
CCIJ2MM-138G
A920CY-100M
CR16-103JM
CR16-8873FM
CR16-1004FM
CR16-4123FM
CR16-2803FM
LTC1877/8EMS8
DESCRIPTION
20pF 50V Chip NPO Capacitor
220pF 50V Chip NPO Capacitor
0.01µF 50V Chip X7R Capacitor
10µF 10V Chip X7R Capacitor
22µF 6.3V Chip X5R Capacitor
47µF 6V POSCAP Capacitor
1µF 10V Chip Y5V Capacitor
Turret, Testpoint
0.079” Single Row Header
0.079” Double Row Header
0.079” Center Shunt
10µH 20% Inductor
10K 5% 1/16W Chip Resistor
887K 1% 1/16W Chip Resistor
1M 1% 1/16W Chip Resistor
412K 1% 1/16W Chip Resistor
280K 1% 1/16W Chip Resistor
Monolithic Synchronous Step-Down Regulator
Optional
Optional
VENDOR
AVX
AVX
AVX
TAIYO-YUDEN
TAIYO-YUDEN
SANYO
TAIYO-YUDEN
Mill-Max
COMM CON
COMM CON
COMM CON
TOKO
TAD
TAD
TAD
TAD
TAD
LTC
TELEPHONE
(843) 946-0362
(843) 946-0362
(843) 946-0362
(408) 573-4150
(408) 573-4150
(619) 661-6835
(408) 573-4150
(516) 922-6000
(626) 301-4200
(626) 301-4200
(626) 301-4200
(847) 699-3430
(714) 255-9123
(714) 255-9123
(714) 255-9123
(800) 508-1521
(800) 508-1521
(408) 432-1900
QUICK START GUIDE
This demonstration board is easily set up to evaluate the
performance of the LTC1877 or LTC1878 IC. Please
follow the procedure outlined below for proper opera-
tion.
• Refer to Figure 5 for proper connection of monitoring
equipment to ensure correct measurement.
• Connect the input power supply to the V
IN
and GND
terminals on the left-hand side of the board. Do not
increase V
IN
over its rated maximum supply voltage or
the part will be damaged. For the LTC1877 the maxi-
mum V
IN
is 10V and for the LTC1878 the maximum
V
IN
is 6V.
• Connect the load between the V
OUT
and GND terminals
on the right side of the board.
• Select the desired operating mode using JP1 and JP2,
as shown in Table 1. JP1 connects SYNC/MODE to
GND in the upper position, and connects it to V
IN
in the
lower position. JP2 connects RUN to GND in the upper
position, and connects it to V
IN
in the lower position.
If a signal is applied at RUN (E1) or SYNC/MODE (E7),
then jumper JP1 or JP2, respectively, must be re-
moved.
• Set the desired output voltage with jumper JP3 as
shown in Figure 2.
Table 1. Operating Mode Selection With Jumpers JP1 and JP2
JP2
JP1
OPERATING MODE
UPPER
LOWER
LOWER
LOWER
OPEN
X
UPPER
LOWER
OPEN
X
SHUTDOWN
PULSE SKIPPING
BURST MODE
EXTERNAL CLOCK AT SYNC/MODE
EXTERNAL SIGNAL AT RUN
JP3
1.5V 2.5V 3.3V OPEN
Figure 2. Output Voltage Selection (JP3)
(3.3V Position Shown)
3
DEMO MANUAL DC290
NO-DESIGN SWITCHER
OPERATIO
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
4
U
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 10µA. 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.
DEMO MANUAL DC290
NO-DESIGN SWITCHER
OPERATIO
BURST
DEFEAT
PLL LPF
8
SYNC/MODE
7
X
0.6V
3
V
FB
–
+
FREQ
SHIFT
0.8V
V
IN
RUN
1
0.8V REF
0.85V
SHUTDOWN
–
OVDET
+
I
RCMP
Figure 3. Functional Block Diagram
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator is reduced to about 80kHz, one-seventh of the
nominal frequency. This frequency foldback ensures that
the inductor current has more time to decay, thereby
preventing runaway. The oscillator’s frequency will
progressively increase to 550kHz (or the synchronized
frequency) when V
FB
rises above 0.3V.
Frequency Synchronization
A phase-locked loop (PLL) is available on the LTC1877
and the LTC1878 to allow the oscillator to be synchronized
to an external source connected to the SYNC/MODE pin.
The output of the phase detector at the PLL LPF pin
operates over a 0V to 2.4V range, corresponding to
400kHz to 700kHz. When locked, the PLL aligns the
turn-on of the MOSFETs to the rising edge of the synchro-
nizing signal.
When the LTC1877 or the LTC1878 is clocked by an
external source, Burst Mode operation is disabled; the
LTC1877 or the LTC1878 then operates in PWM pulse
skipping mode. In this mode, when the output load is very
low, the current comparator, I
COMP
, may remain tripped
for several cycles and force the main switch to stay off for
the same number of cycles. Increasing the output load
slightly allows constant frequency PWM operation to
resume.
Frequency synchronization is inhibited when the feedback
voltage, V
FB
, is below 0.6V. This prevents the external
clock from interfering with the frequency foldback for
short-circuit protection.
+
–
–
+
U
V
IN
Y
Y = “0” ONLY WHEN X IS A CONSTANT “1”
SLOPE
COMP
VCO
OSC
0.8V
6 V
IN
–
+
EA
V
IN
SLEEP
V
IN
2 I
TH
S
R
0.45V
–
+
EN
SLEEP
–
BURST
Q
Q
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
I
COMP
+
6Ω
RS LATCH
ANTI
SHOOT-
THRU
5 SW
4 GND
dc290A BD
5
MCU
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