Final Electrical Specifications
LT1612
Synchronous, Step-Down
800kHz PWM
DC/DC Converter
April 2000
FEATURES
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DESCRIPTION
The LT
®
1612 is an 800kHz, synchronous step-down DC/
DC converter that operates from an input voltage as low
as 2V. Internal 0.45Ω switches deliver output currents up
to 500mA, and the 800kHz switching frequency allows the
use of small, low value ceramic input and output capaci-
tors. Input voltage ranges from 5.5V down to 2V and
output voltage can be set as low as the 620mV reference.
The device features Burst Mode
TM
operation, keeping
efficiency high at light loads. Burst Mode operation can be
defeated by pulling the MODE pin high, enabling constant
switching throughout the load range for low noise.
No-load quiescent current is 160µA and shutdown current
is less than 1µA. The device is available in 8-lead SO and
MSOP packages.
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
Operates from Input Voltage As Low As 2V
Internal 0.7A Synchronous Switches
Uses Ceramic Input and Output Capacitors
620mV Reference Voltage
800kHz Fixed Frequency Switching
Programmable Burst Mode Operation
APPLICATIONS
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Portable Devices
Lithium-Ion Step-Down Converters
5V to 3.3V Conversion
2-Cell Alkaline Step-Down Converters
TYPICAL APPLICATION
0.1µF
V
IN
2V
V
IN
SHDN
LT1612
C1
10µF
MODE
V
C
33k
330pF
FB
GND
R2
232k
1%
R1
215k
1%
C2
68µF
3.15V
BOOST
SW
100pF
L1
10µH
V
OUT
1.2V
500mA
Efficiency for LT1612 vs Linear Regulator
V
OUT
= 1.2V
90
80
V
IN
= 2V
V
IN
= 3V
V
IN
= 2V (LINEAR)
EFFICIENCY (%)
70
60
50
40
V
IN
= 3V (LINEAR)
10
100
LOAD CURRENT (mA)
500
1612 • F01b
C1: TAIYO-YUDEN JMK325BJ106MN
C2: PANASONIC EEFCDOF680R
L1: SUMIDA CD43-100
1612 F01a
30
Figure 1. 2V to 1.2V Converter
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no represen-
tation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
U
U
U
1
LT1612
ABSOLUTE
MAXIMUM
RATINGS
Supply Voltage (V
IN
) ............................................... 5.5V
SW Pin Voltage ....................................................... 5.5V
FB Pin Voltage ............................................... V
IN
+ 0.3V
V
C
Pin Voltage ........................................................... 2V
SHDN Pin Voltage ................................................... 5.5V
MODE Pin Voltage .................................................. 5.5V
PACKAGE/ORDER INFORMATION
ORDER PART
NUMBER
TOP VIEW
V
C
FB
V
IN
GND
1
2
3
4
8
7
6
5
SHDN
MODE
BOOST
SW
LT1612EMS8
MS8 PART MARKING
LTMS
MS8 PACKAGE
8-LEAD PLASTIC MSOP
T
JMAX
= 125°C,
θ
JA
= 120°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
I
Q
Quiescent Current
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C, VIN = VSHDN = 3V
CONDITIONS
MODE = 5V
MODE = 0V, Not Switching
SHDN = 0V
q
q
q
q
V
FB
FB Voltage
FB Line Regulation
FB Pin Bias Current (Note 3)
q
q
g
m
Error Amplifier Transconductance
Min Input Voltage
Max Input Voltage
2
f
OSC
Oscillator Frequency
q
f
OSC
Line Regulation
Maximum Duty Cycle
q
Shutdown Threshold
Minimum Voltage for Active
Maximum Voltage for Shutdown
2
U
U
W
W W
U
W
(Note 1)
BOOST Pin Voltage ....................................... V
IN
+ 5.5V
Junction Temperature ........................................... 125°C
Operating Temperature Range (Note 2) ... –40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
TOP VIEW
V
C
1
FB 2
V
IN
3
GND 4
8
7
6
5
SHDN
MODE
BOOST
SW
ORDER PART
NUMBER
LT1612ES8
S8 PART MARKING
1612
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 120°C/ W
MIN
TYP
1
160
MAX
2
220
1
0.635
0.635
0.15
50
UNITS
mA
µA
µA
V
V
%/V
nA
µmhos
V
V
kHz
kHz
%/ V
%
%
0.605
0.60
0.62
0.62
0.02
7
250
5.5
700
550
85
80
2
0.2
800
1
90
900
1100
q
q
V
V
LT1612
ELECTRICAL CHARACTERISTICS
SYMBOL PARAMETER
SHDN Pin Current
BOOST Pin Current
Switch Current Limit (Note 4)
The
q
denotes specifications which apply over the full operating
temperature range, otherwise specifications are TA = 25°C, VIN = VSHDN = 3V
CONDITIONS
SHDN = 2V
SHDN = 5V
BOOST = V
IN
+ 2V
Duty Cycle = 0%
MODE = OV
MODE = 5V
Burst Mode Operation Current Limit
Switch Voltage Drop
Rectifier Voltage Drop
SW Pin Leakage
MODE = 0V
I
SW
= 500mA
I
RECT
= 500mA
V
SW
= 5V
q
q
q
MIN
TYP
10
30
4
MAX
15
45
900
900
UNITS
µA
µA
mA
mA
mA
mA
600
550
710
650
180
200
300
280
400
1
mV
mV
µA
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
The LT1612E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Bias current flows out of the FB pin.
Note 4:
Duty cycle affects current limit due to slope compensation.
PIN FUNCTIONS
V
C
(Pin 1):
Compensation Pin. This is the current sink/
source output of the error amplifier. By connecting an RC
network from this pin to ground, frequency response can
be tuned for a wide range of circuit configurations. The
voltage at this pin also sets the current limit, and if
grounded, the switch will remain in the OFF state.
FB (Pin 2):
Feedback Pin. This pin is the negative input to
the error amplifier. Connect the resistor divider tap to this
point which sets V
OUT
according to:
V
OUT
= 0.62V (1 + R1/R2)
V
IN
(Pin 3):
Supply Pin. Bypass capacitor C1 must be right
next to this pin.
GND (Pin 4):
Ground Pin. Connect directly to local ground
plane.
SW (Pin 5):
Switch Pin. Connect inductor and boost
capacitor here. Minimize trace area at this pin to keep EMI
down.
BOOST (Pin 6):
This is the supply pin for the switch driver
and must be above V
IN
by 1.5V for proper switch opera-
tion. Connect the boost capacitor to this pin.
MODE (Pin 7):
Burst Mode Operation Disable Pin. For
continuous switching operation (low noise), pull this pin
above 2V. For Burst Mode operation which gives better
light load efficiency, tie to ground. Output ripple voltage in
Burst Mode operation is typically 30mV
P-P
. See applica-
tions section for more information about this function.
SHDN (Pin 8):
Shutdown Pin. Pull this pin low for shut-
down mode. Tie to a voltage between 2V and 5.5V for
normal operation.
U
U
U
3
LT1612
BLOCK DIAGRA
V
IN
3
V
C
1
FB 2
A1
0.62V
SWITCH
SWITCH
DRIVER
5 SW
MODE
7
0.7V
SHDN
8
SHUTDOWN
OPERATIO
The LT1612 employs fixed frequency, current mode con-
trol. This type of control uses two feedback loops. The
main control loop sets output voltage and operates as
follows: A load step causes V
OUT
and the FB voltage to be
perturbed slightly. The error amplifier responds to this
change in FB by driving the V
C
pin either higher or lower.
Because switch current is proportional to the V
C
pin
voltage, this change causes the switch current to be
adjusted until V
OUT
is once again satisfied. Loop compen-
sation is taken care of by an RC network from the V
C
pin
to ground.
Inside this main loop is another that sets current limit on
a cycle-by-cycle basis. This loop utilizes current compara-
tor A2 to control peak current. The oscillator runs at
800kHz and issues a set pulse to the flip-flop at the
beginning of each cycle, turning the switch on. With the
switch now in the ON state the SW pin is effectively
connected to V
IN
. Current ramps up in the inductor linearly
4
W
R
SENSE
0.08Ω
BOOST DIODE
6 BOOST
+
V/I
A2
–
U
+
–
A3
FLIP-FLOP
R
ENABLE
S
Q
RECTIFIER
DRIVE
RECTIFIER
–
+
OSCILLATOR
SLOPE
COMPENSATION
4 GND
1612 BD
at a rate of (V
IN
– V
OUT
)/L. Switch current is set by the V
C
pin voltage and when the voltage across R
SENSE
trips the
current comparator, a reset pulse will be generated and the
switch will be turned off. Since the inductor is now loaded
up with current, the SW pin will fly low and trigger the
rectifier to turn on. Current will flow through the rectifier
decreasing at a rate of V
OUT
/L until the oscillator issues a
new set pulse, causing the cycle to repeat.
If the load is light and V
C
decreases below A3’s trip point,
the device will enter the Burst Mode operation region (the
MODE pin must be at ground or floating). In this state the
oscillator and all other circuitry except the reference and
comparator A3 are switched on and off at low frequency.
This mode of operation increases efficiency at light loads
but introduces low frequency voltage ripple at the output.
For continuous switching and no low frequency output
voltage ripple, pull the MODE pin high. This will disable
comparator A3 which forces the oscillator to run
continuously.
LT1612
OPERATIO
Layout Hints
The LT1612 switches current at high speed, mandating
careful attention to layout for proper performance.
You
will not get advertised performance with careless layouts.
Figure 2 shows recommended component placement for
a buck (step-down) converter. Follow this closely in your
PC layout. Note the direct path of the switching loops.
Input capacitor C1 must be placed close (< 5mm) to the IC
package. As little as 10mm of wire or PC trace from C
IN
to
V
IN
will cause problems such as inability to regulate or
oscillation.
The ground terminal of input capacitor C1 should tie close
to Pin 4 of the LT1612. Doing this reduces dI/dt in the
ground copper which keeps high frequency spikes to a
minimum. The DC/DC converter ground should tie to the
PC board ground plane at one place only, to avoid intro-
ducing dI/dt in the ground plane.
R1
R2
R
C
V
IN
C1
MULTIPLE
VIAs
1612 F02
Figure 2. Recommended Component Placement. Traces
Carrying High Current are Direct. Trace Area at FB Pin and V
C
Pin Is Kept Low. Lead Length to Battery Should Be Kept Short
Burst Mode Operation Defeat
To maintain high efficiency at light loads, the LT1612 will
automatically shift into Burst Mode operation (MODE = 0V
or floating). In this mode of operation the oscillator and
switch drive circuitry is alternately turned on and off,
reducing quiescent current to 160µA. This reduces power
U
consumption but also adds low frequency voltage ripple to
the output. Figure 3 shows switching waveforms for a 5V
to 3.3V converter running in Burst Mode operation. Output
voltage ripple is approximately 20mV
P-P
. If the MODE pin
is pulled high, Burst Mode operation will be inhibited and
the oscillator runs continuously with no low frequency
ripple at the output. See Figures 4 and 5.
V
OUT
20mV/DIV
AC COUPLED
IL
200mA/DIV
5µs/DIV
1612 F03
Figure 3. Output Voltage Ripple is 20mV
P-P
for
the Circuit of Figure 1
C
C
1
2
3
4
LT1612
8
7
6
5
C3
SHDN
MODE
V
OUT
200mV/DIV
AC COUPLED
I
L
200mA/DIV
I
LOAD
10mA TO 310mA
C2
L1
0.1ms/DIV
1612 F04
Figure 4. Transient Response for the Circuit of Figure 1
with the MODE Pin Tied to Ground or Floating
GND
V
OUT
V
OUT
200mV/DIV
AC COUPLED
I
L
200mA/DIV
I
LOAD
10mA TO 300mA
0.1ms/DIV
1612 F05
Figure 5. With the MODE Pin Tied High, Low
Frequency Output Voltage Ripple Is No Longer Present
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stm32/stm8
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