DEMO MANUAL DC232
NO-DESIGN SWITCHER
DESCRIPTIO
LTC1622 Low Input Voltage
Current Mode Step-Down
DC/DC Converter
out, the external P-channel MOSFET is turned on continu-
ously (100% duty cycle), providing low dropout operation
with V
OUT
≅
V
IN
. To further enhance efficiency at low load
currents, the LTC1622 can be set up for Burst Mode
TM
operation or pulse skipping mode by either connecting the
SYNC/MODE pin to V
IN
or leaving it disconnected. The
LTC1622 can also be synchronized to an external clock at
frequencies of up to 750kHz. The LTC1622 is capable of
operating down to a 2V input voltage before the
undervoltage lockout feature is activated. Soft start is
provided by an external capacitor, which can also be used
to properly sequence supplies.
Gerber files for this circuit
board are available. Call the LTC factory.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
Demo board DC232 is a step-down (buck) regulator using
the LTC1622. The exclusive use of surface mount compo-
nents results in a highly efficient application in a very small
board space. It is ideal for cell phones and other portable
electronics operating from one or two Li-Ion cells or three
to six NiCd cells. DC232 is capable of providing 1.5A at an
output voltage of 2.5V with an input supply of 4.2V. This
demo board highlights the capabilities of the LTC1622,
which uses a current mode PWM architecture to drive an
external P-channel power MOSFET. The results are a high
performance power supply that has low output voltage
ripple. Constant operating frequency makes the LTC1622
attractive for noise-sensitive applications. In addition,
high efficiency over wide load current range makes the
LTC1622 ideal for battery-powered applications. In drop-
PERFOR A CE SU
SYMBOL
V
IN
V
OUT
PARAMETER
Output Voltage
Input Working Voltage Range
V
FB
I
Q
Feedback Voltage
Typical Supply Current
Normal Mode
Shutdown
V
IN
= 4.2V, I
OUT
= 0mA
V
IN
= 4.2V, V
RUN/SS
= 0V
TYPICAL PERFOR A CE CHARACTERISTICS A D BOARD PHOTO
Efficiency vs Load Current
(Burst Mode Enabled)
100
V
IN
= 3.3V
90
V
IN
= 4.2V
90
EFFICIENCY (%)
Efficiency vs Load Current
(Burst Mode Defeated)
100
V
IN
= 3.3V
V
IN
= 4.2V
EFFICIENCY (%)
80
V
IN
= 8.4V
70
60
50
40
1
10
100
LOAD CURRENT (mA)
1000
DC232G01
80
70
60
50
40
1
10
100
LOAD CURRENT (mA)
1000
DC232 BP
DC232G02
V
IN
= 6V
V
IN
= 6V
V
IN
= 8.4V
V
OUT
= 2.5V
R
SENSE
= 0.03Ω
V
OUT
= 2.5V
R
SENSE
= 0.03Ω
U
WW
U W
U
U W
ARY
CONDITIONS
V
OUT
= 1.8V
I
OUT
= 800mA
I
OUT
= 800mA
I
OUT
= 800mA
VALUE
2V to 8.5V
3.3V
±
0.033V
2.5V
±
0.025V
1.8V
±
0.018V
0.8V
±
0.04V
400µA
15µA
Board Photo
1
DEMO MANUAL DC232
NO-DESIGN SWITCHER
PERFOR A CE SU
SYMBOL
I
OUT
∆V
OUT
V
RIPPLE
f
SYNC
PARAMETER
Maximum Output Current
Typical Load Regulation
Typical Output Ripple
Maximum Synchronizable Frequency
PACKAGE A D SCHE ATIC DIAGRA SM
1
2
3
RUN/
SS
R1
10k
SENSE
–
I
TH
V
FB
V
IN
PDRV
U1
LTC1622
GND
8
7
6
D1
SYNC/ 5 MODE/
MODE
SYNC
L1
4.7µH
C2
47µF
6V
R3
237k
R6
158k
A
1.8V
R4
75k
ADJ1
GND
DC232 F01
R2
0.03Ω
C1
10µF
10V
V
IN
2V TO
8.5V
Si3443DV
V
OUT
1,8V, 2.5V,
3.3V
C3
470pF
4 RUN/
SS
+
C5
120pF
C4
470pF
Figure 1. LTC1622 Low Input Voltage Current Mode Step-Down DC/DC Converter Schematic
PARTS LIST
REFERENCE
DESIGNATOR QUANTITY
C1
C2
C3, C4
C5
D1
J1 to J6
JP1
L1
M1
R1
R2
R3
R4
R5
R6
U1
1
1
1
1
1
6
1
1
1
1
1
1
1
1
1
1
PART NUMBER
LMK325BJ106K-T
6TPA47M
06035A471KAT
06035A121KAT
IR10BQ015
1502-2
22025-6G2
LQN6C-4R7
Si3443DV
CR16-103J
LR1206-01-R030F
CR16-2373FM
CR16-7502FM
CR16-4993FM
CR16-1583FM
LTC1622CMS8
DESCRIPTION
10µF 10V Ceramic Capacitor
47µF 6V POSCAP Capacitor
470pF 50V 10% NPO Chip Capacitor
120pF 50V 10% NPO Chip Capacitor
Schottky Diode
0.09
"
Turret Connector
0.079
"
cc 6-Pin Jumper
4.7µH Inductor
Sublogic Threshold P-Channel MOSFET
10k 5% 0.1W 0603 Resistor
0.030Ω 1% 0.25W 1206 Resistor
237k 1% 0.1W 0603 Resistor
75k 1% 0.1W 0603 Resistor
499k 1% 0.1W 0603 Resistor
158k 1% 0.1W 0603 Resistor
8-Pin MSOP IC
VENDOR
Taiyo Yuden
Sanyo
AVX
AVX
International Rectifier
Keystone
Comm Con
Murata
Siliconix
TAD
IRC
TAD
TAD
TAD
TAD
LTC
TELEPHONE
(408) 573-4150
(619) 661-6835
(843) 946-0362
(843) 946-0362
(310) 322-3331
(718) 956-8900
(626) 301-4200
(814) 237-1431
(800) 554-5565
(800) 508-1521
(512) 992-7900
(800) 508-1521
(800) 508-1521
(800) 508-1521
(800) 508-1521
(408) 432-1900
2
W
WW
W
U
U W
ARY
CONDITIONS
V
IN
= 4.2V, V
OUT
= 2.5V
0mA < I
OUT
< 1.5A, V
IN
= 8.5V
I
OUT
= 1.5A, V
IN
= 4.2V, Burst Mode Operation Defeated
I
OUT
= 100mA, V
IN
= 4.2V, Burst Mode Operation Enabled
V
OUT
= 2.5V, V
IN
≤
8.5V
V
OUT
= 1.8V, V
IN
≤
7V
VALUE
1.5A
–1%
40mV
P-P
120mV
P-P
750kHz
600kHz
TOP VIEW
1
I
TH
2
V
FB
3
RUN/SS 4
SENSE
–
8
7
6
5
V
IN
PDRV
GND
SYNC/MODE
MS8 PACKAGE
8-LEAD PLASTIC MSOP
R5
499k
B
2.5V
JP1
ADJ2
C
LTC1622
3.3/ADJ
DEMO MANUAL DC232
NO-DESIGN SWITCHER
QUICK START GUIDE
This demonstration board is easy to set up to evaluate the
performance of the LTC1622. Please follow the proce-
dure outlined below for proper operation.
1. Move jumper JP1 to the appropriate position for the
required output voltage. For voltages other than the
preset value, make sure you install the calculated
resistor at the pads provided (see Output Voltage
Setup).
2. To shut down the circuit, connect the RUN/SS pin to
ground.
3. For synchronized operation, connect the clock signal
between the SYNC/MODE and ground pins. Do not
apply more than the input voltage (V
IN
) on this pin.
4. For Burst Mode operation at low load currents, float
the SYNC/MODE pin or connect it to V
IN
. Grounding or
clocking this pin inhibits Burst Mode operation.
5. Connect the input power supply to the V
IN
and GND
terminals.
6. Connect the load between the V
OUT
and GND termi-
nals. Refer to Figure 4 for proper measurement equip-
ment setup.
OPERATIO
The circuit shown in Figure 1 operates from an input
voltage between 2V and 8.5V. Output voltages of 1.8V,
2.5V and 3.3V can be easily set by moving jumper JP1 to
the appropriate position. For other output voltages, a user-
installable resistor must be connected to the pads pro-
vided (see Output Voltage Setup).
This demonstration circuit has been optimized for effi-
ciency and physical footprint. For other requirements,
please contact the factory. This demonstration circuit is
intended for the evaluation of the LTC1622 switching
regulator IC and was not designed for any other purpose.
OPERATION
The LTC1622 uses the constant-frequency, pulse-width-
modulated, current mode architecture shown in Figure 2.
Current mode operation provides the well known advan-
tages of clean start-up and excellent line and load
regulation.
The LTC1622 is designed to operate down to a 2V input
voltage, making it suitable for applications that are pow-
ered either by a low input supply or a single lithium-ion
battery. The external MOSFET can limit the minimum input
voltage; therefore, be careful when specifying the MOSFET.
To prevent damage to the lithium-ion battery by deep
discharge, an undervoltage lockout circuit is incorporated
into the LTC1622. When the input supply drops to about
U
2V, all circuitry, except the undervoltage detector block, is
turned off .
The LTC1622 operates as follows: the external P-channel
power MOSFET is turned on at the beginning of each cycle
when the oscillator sets the latch (R
S1
), and is turned off
when the current comparator (I
COMP
) resets the latch. The
peak inductor current at which the I
COMP
resets the R
S
latch is controlled by the voltage on the I
TH
pin, which is the
output of the error amplifier EA. An external resistive
divider connected between V
OUT
and ground allows EA to
receive an output feedback voltage, V
FB
. When the load
current increases, it causes a slight decrease in V
FB
relative to the 0.8V reference, which, in turn, causes the I
TH
voltage to increase until the average inductor current
matches the new load current.
The main control loop is shut down by tying the RUN/SS
pin low. Releasing RUN/SS allows an internal 0.75µA
current source to charge the soft start capacitor C4. When
C4 reaches 0.7V, the main control loop is enabled, with the
I
TH
voltage clamped at approximately 5% of its maximum
value. As C4 continues to charge, I
TH
is gradually released,
allowing normal operation to resume.
Comparator OV guards against output transients that are
greater than 8% over the target output voltage by turning
off the P-channel power MOSFET and keeping it off until
the fault is removed.
3
DEMO MANUAL DC232
NO-DESIGN SWITCHER
OPERATIO
V
IN
8
1µA
SYNC/
5
MODE
0.3V
V
FB
3
V
IN
0.8V
REFERENCE
UVLO
TRIP = 1.97V
SHUTDOWN
Burst Mode Operation
The LTC1622 can be enabled for Burst Mode operation at
low load currents by connecting the SYNC/MODE pin to
the input supply or allowing the pin to float. In this mode,
the minimum peak current of the inductor is set to 0.036V/
R
SENSE
(at low duty cycle), even though the voltage at the
I
TH
pin would normally indicate a lower value. If the
inductor’s average current is greater than the load require-
ment, the voltage at the I
TH
pin will drop as V
OUT
increases.
When the I
TH
voltage goes below 0.12V, the sleep signal
goes high, turning off the external MOSFET. The sleep
signal goes low when the I
TH
voltage goes above 0.22V
and the LTC1622 resumes normal operation. The next
oscillator cycle will turn the external MOSFET on and the
switching cycle repeats.
4
U
BURST DEFEAT
X
Y
Y = “0” ONLY WHEN X IS A CONSTANT “1”
OTHERWISE Y = “1”
V
CC
SLOPE
COMP
OSC
0.36V
–
+
–
FREQ
SHIFT
0.8V
EN
SENSE
–
V
IN
1
8
+
+
EA
0.12V
2
RUN/
SOFT START
10k
I
TH
–
+
BURST
S
R
Q
Q
R
S1
SLEEP
+
SWITCHING
LOGIC
AND
BLANKING
CIRCUIT
–
V
IN
RUN/SS 4
0.75µA
I
COMP
–
V
IN
PDRV
7
+
6
GND
OV
V
REF
+ 66mV
–
DC232 F02
Figure 2. LTC1622 Block Diagram
Frequency Synchronization
The LTC1622 can be externally driven by a clock signal. For
output voltages of 2.5V and 3.3V, this demo circuit can be
synchronized to frequencies up to 750kHz. For an output
voltage of 1.8V, it can be synchronized to frequencies of up
to 600kHz.
Do not
synchronize the LTC1622 below its
default operating frequency of 550kHz, as this may cause
abnormal operation and an undesired frequency spec-
trum.
Synchronization is inhibited when the feedback voltage is
below 0.3V. This is to prevent inductor current build-up
under short-circuit conditions. Burst Mode operation is
inhibited when the LTC1622 is driven by an external clock.
DEMO MANUAL DC232
NO-DESIGN SWITCHER
OPERATIO
Undervoltage Lockout
To prevent deep discharge of a lithium-ion battery when it
is near its end of charge, an undervoltage lockout circuit is
incorporated into the LTC1622. When the input supply
voltage drops to 2V, all circuitry is turned off except the
undervoltage block, which draws only several microam-
peres.
Short-Circuit Protection
When the output is shorted to ground, the frequency of the
oscillator will be reduced to about 1/4.5 of its normal rate.
This low frequency allows the inductor current to dis-
charge, preventing runaway. The oscillator’s frequency
will gradually increase to its nominal rate when the feed-
back voltage increases above 0.65V. Note that synchroni-
zation is inhibited until the feedback voltage goes above
0.4V.
Output Voltage Setup
In this demonstration circuit, output voltages of 1.8V, 2.5V
and 3.3V can be obtained by moving jumper JP1 to the
appropriate position, as indicated on the demo board. In
addition, provision has been made for the user to set the
LTC1622 for other voltages by adding a resistor and
moving the jumper to the ADJ position. To obtain voltages
above 1.8V, move jumper JP1 to the A position and install
the required resistance at the pads labelled ADJ1. The
required resistance is given by:
R
ADJ1
(
k
Ω) =
75
.
84
V
OUT
−
1
.
81
0.8V
REF
+
EA
–
LTC1622
R3
75k
ADJ1
Figure 3. Output Voltage Setting
U
Higher output voltages may require a substitute output
capacitor since the installed output capacitor is rated at
only 6V. Normally, we use a capacitor with a voltage rating
of twice the output voltage. For output voltages below
1.8V, move the jumper JP1 to position C and install the
required resistance at the pads labelled ADJ2. The re-
quired resistance is given by:
R
ADJ2
(
k
Ω) =
17775 V
OUT
−
0
.
8
249
.
6
−
75V
OUT
(
)
HOW TO MEASURE VOLTAGE REGULATION
When measuring voltage regulation, all measurements
must be taken at the point of regulation. This point is where
the LTC1622 control loop looks for the information to keep
the output voltage constant. This information occurs
between Pin 3 and Pin 6 of the LTC1622. For output
voltages other than 0.8V, the voltage at Pin 3 can be
adjusted by the resistor divider network. These points
correspond to the output terminals of the demonstration
board. Test leads should be attached to these terminals
and the load should be attached as close to these terminals
as possible. This applies to line regulation (input-to-
output voltage regulation) as well as load regulation tests.
In performing line regulation tests, always look at the input
voltage across the input terminals. Refer to Figure 4 for
proper monitoring equipment configuration.
For the purposes of these tests, the demonstration circuit
should be powered by a regulated DC bench supply so
additional variation on the DC input does not add an error
to the regulation measurements.
V
IN
A
LTC1622
V
OUT
V
OUT
R4
237k
R6
158k
A
B
R5
499k
ADJ2
C
+
A
LOAD
+
+
V
MODE/SYNC
+
RUN/SS
GND
V
GND
DC232 F04
DC232 F03
Figure 4. Correct Measurement Setup
5
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