LTC1626
Low Voltage, High Efficiency
Step-Down DC/DC Converter
FEATURES
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DESCRIPTION
The LTC
®
1626 is a monolithic, low voltage, step-down
current mode DC/DC converter featuring Burst Mode
TM
operation at low output current.
The input supply voltage range of 2.5V to 6V makes the
LTC1626 ideal for single cell Li-Ion and 3- or 4-cell NiCd/
NiMH applications. A built-in 0.32Ω switch (V
IN
= 4.5V)
allows up to 0.6A of output current.
The LTC1626 incorporates automatic power saving Burst
Mode operation to reduce gate charge losses when the
load current drops below the level required for continuous
operation. With no load, the converter draws only 165µA.
In shutdown, it draws a mere 0.5µA—making it ideal for
current sensitive applications.
The inductor current is user-programmable via an external
current sense resistor. In dropout, the internal P-channel
MOSFET switch is turned on continuously, maximizing
battery life.
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Wide Input Supply Voltage Range: 2.5V to 6V
High Efficiency: Up to 95%
Low R
DS(ON)
Internal Switch: 0.32
Ω
(V
IN
= 4.5V)
Current Mode Operation for Excellent Line and Load
Transient Response
Short-Circuit Protected
Low Dropout Operation: 100% Duty Cycle
Built-In Low-Battery Detector
Low Quiescent Current at Light Loads: I
Q
= 165µA
Ultralow Shutdown Current: I
Q
= 0.5µA
Peak Inductor Current Independent of Inductor Value
Available in 14-Pin SO Package
APPLICATIONS
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Single Cell Li-Ion Step-Down Converters
3- or 4-Cell NiMH Step-Down Converters
Cellular Telephones
5V to 3.3V Conversion
3.3V to 2.5V Conversion
Inverting Converters
Portable Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
V
IN
2.7V TO 6V
+
C
IN†
47µF
16V
0.1µF
PWR V
IN
SHDN
LTC1626
V
IN
SW
PGND
470Ω
I
TH
C
T
SENSE
+
SENSE
–
SGND
V
FB
100pF
10k
1626 F01
100
L*
33µH
D1
MBRS130LT
R
SENSE
**
0.1Ω
V
OUT
2.5V
0.25A
95
3900pF
+
C
OUT††
100µF
6.3V
EFFICIENCY (%)
90
85
80
75
70
0.01
1000pF
10k
C
T
270pF
* COILTRONICS CTX33-4
** IRC 1206-R100F
†
AVX TPSD476KO16
††
AVX TPSC107M006R0150
Figure 1. High Efficiency 2.5V Step-Down Converter
U
U
U
Efficiency
V
IN
= 3.5V
L1 = 33µH
V
OUT
= 2.5V
R
SENSE
= 0.1Ω
C
T
= 270pF
0.1
OUTPUT CURRENT (A)
1
1626 F01a
1
LTC1626
ABSOLUTE
MAXIMUM
RATINGS
(Voltages Referred to GND Pin)
Input Supply Voltage (Pins 1, 2, 13) ............– 0.3V to 7V
Shutdown Input Voltage (Pin 10) ................– 0.3V to 7V
Sense
–
, Sense
+
(Pins 7, 8)........... – 0.3V to (V
IN
+ 0.3V)
LBO, LBI (Pins 3, 4) .................................... – 0.3V to 7V
C
T
, I
TH
, V
FB
(Pins 5, 6, 9) ............. – 0.3V to (V
IN
+ 0.3V)
DC Switch Current (Pin 14) .................................... 1.2A
Peak Switch Current (Pin 14) ................................. 1.6A
Switch Voltage (Pin 14) .......(V
IN
– 7.5V) to (V
IN
+ 0.3V)
Operating Temperature Range ..................... 0°C to 70°C
Extended Commercial Operating
Temperature Range (Note 4) ............. – 40°C to 85°C
Junction Temperature (Note 1) ............................. 125°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
PACKAGE/ORDER INFORMATION
TOP VIEW
PWR V
IN
1
V
IN
2
LBO 3
LBI 4
C
T
5
I
TH
6
SENSE
–
7
14 SW
13 PWR V
IN
12 PGND
11 SGND
10 SHDN
9 V
FB
8 SENSE
+
ORDER PART
NUMBER
LTC1626CS
S PACKAGE
14-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 110°C/ W
Consult factory for Industrial and Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
FB
V
FB
∆V
OUT
PARAMETER
Feedback Pin Current
Feedback Voltage
Output Voltage Line Regulation
Output Voltage Load Regulation
Burst Mode Output Ripple
I
Q
Input DC Supply Current (Note 2)
Active Mode
Sleep Mode
Shutdown
Low-Battery Trip Point
Low-Battery Input Bias Current
Low-Battery Output Sink Current
Current Sense Threshold Voltage
V
SENSE +
– V
SENSE–
ON Resistance of Switch
Switch Off-Time (Note 3)
SHDN Pin High
SHDN Pin Low
SHDN Pin Input Current
V
LBO
= 0.4V
CONDITIONS
T
A
= 25°C, V
IN
= 4.5V, V
OUT
= 2.5V, V
SHDN
= 0V, unless otherwise specified.
MIN
q
q
TYP
0.1
1.25
0
25
50
MAX
1
1.28
1.3
40
50
UNITS
µA
V
V
mV
mV
mV
P-P
mA
µA
µA
V
µA
mA
mV
mV
Ω
µs
V
V
µA
0°C to 70°C
– 40°C to 85°C
V
IN
= 3.5V to 5.5V, I
LOAD
= 250mA
10mA
≤
I
LOAD
≤
250mA
I
LOAD
= 0
1.22
1.2
– 40
q
q
V
SHDN
= V
IN
1.15
0.4
130
4
V
IN
– 0.4
1.9
165
0.5
1.25
1.4
25
155
0.32
5
3.0
300
5
1.35
±0.5
V
LBTRIP
I
LBI
I
LBO
V
SENSE
R
ON
t
OFF
V
IHSD
V
ILSD
I
INSD
V
SENSE –
= 2.5V, V
FB
= V
OUT
/2 + 25mV (Forced)
V
SENSE –
= 2.5V, V
FB
= V
OUT
/2 – 25mV (Forced)
C
T
= 390pF, I
LOAD
= 400mA
Minimum Voltage for Device to Be Shut Down
Maximum Voltage for Device to Be Active
0V
≤
V
SHDN
≤
7V
q
180
0.45
6
0.4
±1
The
q
denotes specifications that apply over the specified operating
temperature range.
Note 1:
T
J
is calculated from the ambient temperature T
A
and power
dissipation according to the following formula:
T
J
= T
A
+ (P
D
• 110°C/W)
Note 2:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency.
Note 3:
In applications where R
SENSE
is placed at ground potential, the
off-time increases by approximately 40%.
Note 4:
C grade device specifications are guaranteed over the 0°C to 70°C
temperature range. In addition, C grade device specifications are assured
over the – 40°C to 85°C temperature range by design or correlation, but
are not production tested.
2
U
W
U
U
W W
W
LTC1626
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Input Voltage
(V
OUT
= 2.5V)
100
98
96
EFFICIENCY (%)
L1 = 33µH
R
SENSE
= 0.1Ω
C
T
= 270pF
I
OUT
= 100mA
EFFICIENCY (%)
EFFICIENCY (%)
94
92
90 I
OUT
= 250mA
88
86
84
82
80
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
1626 G01
Operating Frequency
2.0
1.8
FIGURE 1 CIRCUIT
1.0
0.9
NORMALIZED FREQUENCY
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
1626 G04
0.7
LEAKAGE CURRENT (µA)
1.6
R
DS(ON)
(Ω)
DC Supply Current*
5.0
4.5
4.0
T
J
= 25°C
* DOES NOT INCLUDE
GATE CHARGE CURRENT
0.50
0.45
0.40
SUPPLY CURRENT (mA)
SUPPLY CURRENT (µA)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
SLEEP MODE
ACTIVE MODE
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
1626 G08
OUTPUT VOLTAGE (V)
0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
1626 G07
U W
Efficiency vs Output Current
(V
OUT
= 3.3V)
100
95
90
85
80
75
L1 = 33µH
V
IN
= 5V
V
OUT
= 3.3V
R
SENSE
= 0.1Ω
C
T
= 270pF
0.1
OUTPUT CURRENT (A)
1
1626 G02
Efficiency vs Input Voltage
(V
OUT
= 3.3V)
100
98
96
94
92
90
88
86
84
82
L1 = 33µH
R
SENSE
= 0.1Ω
C
T
= 270pF
I
OUT
= 250mA
I
OUT
= 100mA
70
0.01
80
2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
INPUT VOLTAGE (V)
1626 G03
Switch Resistance
100
90
80
70
60
50
40
30
20
10
0
Switch Leakage Current
V
IN
= 4.5V
0.8
0.6
0.5
0.4
0.3
0.2
0.1
0
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
INPUT VOLTAGE (V)
1626 G05
T
J
= 70°C
T
J
= 25°C
T
J
= 0°C
0
10 20 30 40 50 60 70 80 90 100
JUNCTION TEMPERATURE (°C)
1626 G06
Supply Current in Shutdown
5.0
T
J
= 25°C
SHUTDOWN = V
IN
Low Voltage Behavior
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
INPUT VOLTAGE (V)
1626 G09
L1 = 33µH
R
SENSE
= 0.1Ω
C
T
= 270pF
T
J
= 25°C
I
LOAD
= 250mA
V
OUT
= 3.3V
V
OUT
= 2.5V
3
LTC1626
PIN FUNCTIONS
PWR V
IN
(Pins 1, 13):
Supply for the Power MOSFET and
Its Driver. Decouple this pin properly to ground.
V
IN
(Pin 2):
Main Supply for All the Control Circuitry in
the LTC1626.
LBO (Pin 3):
Open-Drain Output of the Low-Battery Com-
parator. This pin will sink current when Pin 4 (LBI) goes
below 1.25V. During shutdown, this pin is high imped-
ance.
LBI (Pin 4):
The (–) Input of the Low-Battery Comparator.
The (+) input is connected to a reference voltage of 1.25V.
If not used, connect to V
IN
.
C
T
(Pin 5):
External capacitor C
T
from Pin 5 to ground sets
the switch off-time. The operating frequency is dependent
on the input voltage and C
T
.
I
TH
(Pin 6):
Feedback Amplifier Decoupling Point. The
current comparator threshold is proportional to Pin 6
voltage.
SENSE
–
(Pin 7):
Connects to the (–) Input of the Current
Comparator.
SENSE
+
(Pin 8):
The (+) Input to the Current Comparator.
A built-in offset between Pins 7 and 8 in conjunction with
R
SENSE
sets the current trip threshold.
V
FB
(Pin 9):
This pin serves as the feedback pin from an
external resistive divider used to set the output voltage.
SHDN (Pin 10):
Shutdown Pin. Pulling this pin to V
IN
keeps the internal switch off and puts the LTC1626 in
micropower shutdown. If not used, connect to SGND.
SGND (Pin 11):
Small-Signal Ground. Must be routed
separately from other grounds to the (–) terminal of C
OUT
.
PWR GND (Pin 12):
Switch Driver Ground. Connects to
the (–) terminal of C
IN
.
SW (Pin 14):
Drain of the P-Channel MOSFET Switch.
Cathode of the Schottky diode must be connected closely
to this pin.
BLOCK DIAGRAM
SLEEP
+
S
–
V
TH2
V
TH1
5 C
T
4
W
U
U
U
PWR V
IN
1
13
SENSE
+
8
SENSE
–
7
PWR GND
12
14 SW
–
V
9
V
FB
+
R
Q
S
I
TH
6
–
C
+
25mV TO 150mV
+
13k
G
V
OS
–
+
+
REFERENCE
10 SHDN
4 LBI
1626 BD
–
T
V
IN
2
OFF-TIME
CONTROL
LBO
3
A3
+
SENSE
–
V
FB
SGND 11
–
LTC1626
OPERATIO
The nominal off-time of the LTC1626 is set by an external
timing capacitor connected between the C
T
pin and ground.
The operating frequency is then determined by the off-
time and the difference between V
IN
and V
OUT
.
The output voltage is set by an external divider returned to
the V
FB
pin. A voltage comparator V and a gain block G
compare the divided output voltage with a reference
voltage of 1.25V.
To optimize efficiency, the LTC1626 automatically switches
between continuous and Burst Mode operation. The volt-
age comparator is the primary control element when the
device is in Burst Mode operation, while the gain block
controls the output voltage in continuous mode.
When the load is heavy, the LTC1626 is in continuous
operation. During the switch “ON” time, current compara-
tor C monitors the voltage between the SENSE
+
and
SENSE
–
pins connected across an external shunt in series
with the inductor. When the voltage across the shunt
reaches the comparator’s threshold value, its output sig-
nal changes state, resetting the flip-flop and turning the
internal P-channel MOSFET off. The timing capacitor
connected to the C
T
pin is now allowed to discharge at a
rate determined by the off-time controller.
When the voltage on the timing capacitor has discharged
past V
TH1
, comparator T trips, sets the flip-flop and causes
the switch to turn on. Also, the timing capacitor is
recharged. The inductor current will again ramp up until
the current comparator C trips. The cycle then repeats.
When the load current increases, the output voltage
APPLICATIONS INFORMATION
The basic LTC1626 application circuit is shown in Figure
1. External component selection is driven by the load
requirement and begins with the selection of R
SENSE
. Once
R
SENSE
is known, C
T
and L can be chosen. Next, the
Schottky diode D1 is selected followed by C
IN
and C
OUT
.
R
SENSE
Selection for Output Current
R
SENSE
is chosen based on the required output current.
With the current comparator monitoring the voltage devel-
oped across R
SENSE
, the threshold of the comparator
determines the peak inductor current. Depending upon
the load current condition, the threshold of the compara-
tor lies between 25mV/R
SENSE
and 150mV/R
SENSE
. The
maximum output current of the LTC1626 is:
I
OUT(MAX)
= 150mV/R
SENSE
– I
RIPPLE
/2 (A)
Where I
RIPPLE
is the peak-to-peak inductor ripple current.
At a relatively light load, the LTC1626 is in Burst Mode
operation. In this mode, the peak current is set at 25mV/
R
SENSE
. To fully benefit from Burst Mode operation, the
U
W
U
U
U
decreases slightly. This causes the output of the gain stage
(Pin 6) to increase the current comparator threshold, thus
tracking the load current.
When the load is relatively light, the LTC1626 automati-
cally switches to Burst Mode operation. The current loop
is interrupted when the output voltage reaches the desired
regulated value. The hysteretic voltage comparator V trips
when V
OUT
is above the desired output voltage, turning off
the switch and causing the timing capacitor to discharge.
This capacitor discharges past V
TH1
until its voltage drops
below V
TH2
. Comparator S then trips and a sleep signal is
generated. The circuit now enters into sleep mode with the
power MOSFET turned off. In sleep mode, the LTC1626 is
in standby and the load current is supplied by the output
capacitor. All unused circuitry is shut off, reducing quies-
cent current from 1.9mA to 165µA. When the output
capacitor discharges by the amount of the hysteresis of
the comparator V, the P-channel switch turns on again and
the process repeats itself. During Burst Mode operation,
the peak inductor’s current is set at 25mV/R
SENSE
.
To avoid the operation of the current loop interfering with
Burst Mode operation, a built-in offset V
OS
is incorporated
in the gain stage. This prevents the current from increas-
ing until the output voltage has dropped below a minimum
threshold.
In dropout, the P-channel MOSFET is turned on continu-
ously (100% duty cycle) providing low dropout operation
with V
OUT
≅
V
IN
.
5
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