LTC1624
High Efficiency SO-8
N-Channel Switching
Regulator Controller
FEATURES
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DESCRIPTION
The LTC
®
1624 is a current mode switching regulator
controller that drives an external N-channel power MOSFET
using a fixed frequency architecture. It can be operated in
all standard switching configurations including boost,
step-down, inverting and SEPIC. Burst Mode
TM
operation
provides high efficiency at low load currents. A maximum
high duty cycle limit of 95% provides low dropout operation
which extends operating time in battery-operated systems.
The operating frequency is internally set to 200kHz, allowing
small inductor values and minimizing PC board space. The
operating current level is user-programmable via an external
current sense resistor. Wide input supply range allows
operation from 3.5V to 36V (absolute maximum).
A multifunction pin (I
TH
/ RUN) allows external
compensation for optimum load step response plus
shutdown. Soft start can also be implemented with the
I
TH
/RUN pin to properly sequence supplies.
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N-Channel MOSFET Drive
Implements Boost, Step-Down, SEPIC
and Inverting Regulators
Wide V
IN
Range: 3.5V to 36V Operation
Wide V
OUT
Range: 1.19V to 30V in Step-Down
Configuration
±
1% 1.19V Reference
Low Dropout Operation: 95% Duty Cycle
200kHz Fixed Frequency
Low Standby Current
Very High Efficiency
Remote Output Voltage Sense
Logic-Controlled Micropower Shutdown
Internal Diode for Bootstrapped Gate Drive
Current Mode Operation for Excellent Line and
Load Transient Response
Available in an 8-Lead SO Package
APPLICATIONS
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Notebook and Palmtop Computers, PDAs
Cellular Telephones and Wireless Modems
Battery-Operated Digital Devices
DC Power Distribution Systems
Battery Chargers
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATION
1000pF
SENSE
–
I
TH
/RUN
V
FB
100pF
GND
SW
V
IN
BOOST
TG
V
IN
4.8V TO 28V
R
SENSE
0.05Ω
M1
Si4412DY
C
B
0.1µF
D1
MBRS340T3
L1
10µH
C
C
470pF
R
C
6.8k
LTC1624
R2
35.7k
R1
20k
Figure 1. High Efficiency Step-Down Converter
U
U
U
+
C
IN
22µF
35V
×
2
V
OUT
3.3V
2A
+
C
OUT
100µF
10V
×
2
1624 F01
1
LTC1624
ABSOLUTE
MAXIMUM
RATINGS
Input Supply Voltage (V
IN
)......................... 36V to – 0.3V
Topside Driver Supply Voltage (BOOST)....42V to – 0.3V
Switch Voltage (SW).................................. 36V to – 0.6V
Differential Boost Voltage
(BOOST to SW) ....................................7.8V to – 0.3V
SENSE
–
Voltage
V
IN
< 15V .................................. (V
IN
+ 0.3V) to – 0.3V
V
IN
≥
15V .......................... (V
IN
+0.3V) to (V
IN
– 15V)
I
TH
/RUN, V
FB
Voltages ............................ 2.7V to – 0.3V
Peak Driver Output Current < 10µs (TG) .................... 2A
Operating Temperature Range
LTC1624CS ............................................ 0°C to 70°C
LTC1624IS ......................................... – 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
SENSE
–
1
I
TH
/RUN 2
V
FB
3
GND 4
8
7
6
5
V
IN
BOOST
TG
SW
ORDER PART
NUMBER
LTC1624CS8
LTC1624IS8
S8 PART MARKING
1624
1624I
S8 PACKAGE
8-LEAD PLASTIC SO
T
JMAX
= 125°C,
θ
JA
= 110°C/ W
Consult factory for Military grade parts.
ELECTRICAL CHARACTERISTICS
SYMBOL
I
IN
V
FB
V
FB
∆V
LINE REG
∆V
LOAD REG
PARAMETER
Feedback Current
Feedback Voltage
Reference Voltage Line Regulation
Output Voltage Load Regulation
Main Control Loop
T
A
= 25°C, V
IN
= 15V, unless otherwise noted.
MIN
TYP
10
q
CONDITIONS
(Note 2)
(Note 2)
V
IN
= 3.6V to 20V (Note 2)
(Note 2)
I
TH
Sinking 5µA
I
TH
Sourcing 5µA
(Note 3)
V
ITH/RUN
= 0V
q
q
MAX
50
1.2019
0.01
0.8
– 0.8
1.32
900
30
– 5.0
– 350
185
150
150
225
5.5
5
UNITS
nA
V
%/V
%
%
V
µA
µA
V
µA
µA
mV
ns
ns
kHz
V
%
1.1781
1.19
0.002
0.5
– 0.5
V
OVL
I
Q
Output Overvoltage Lockout
Input DC Supply Current
Normal Mode
Shutdown
Run Threshold
Run Current Source
Run Pullup Current
TG Transition Time
Rise Time
Fall Time
Oscillator Frequency
Boost Voltage
Boost Load Regulation
SW = 0V, I
BOOST
= 5mA, V
IN
= 8V
SW = 0V, I
BOOST
= 2mA to 20mA
V
ITH/RUN
= 0.3V
V
ITH/RUN
= 1V
V
FB
= 1.0V
C
LOAD
= 3000pF
C
LOAD
= 3000pF
q
1.24
1.28
550
16
V
ITH/RUN
I
ITH/RUN
0.6
– 0.8
– 50
145
0.8
– 2.5
–160
160
50
50
∆V
SENSE(MAX)
Maximum Current Sense Threshold
TG t
r
TG t
f
f
OSC
V
BOOST
∆V
BOOST
175
4.8
200
5.15
3
The
q
denotes specifications which apply over the full operating
temperature range.
LTC1624CS: 0°C
≤
T
A
≤
70°C
LTC1624IS: – 40°C
≤
T
A
≤
85°C
Note 1:
T
J
is calculated from the ambient temperature T
A
and power
dissipation P
D
according to the following formula:
T
J
= T
A
+ (P
D
• 110°C/W)
Note 2:
The LTC1624 is tested in a feedback loop which servos V
FB
to
the midpoint for the error amplifier (V
ITH
= 1.8V).
Note 3:
Dynamic supply current is higher due to the gate charge being
delivered at the switching frequency. See Applications Information.
2
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LTC1624
TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current
V
OUT
= 3.3V
100
95
EFFICIENCY (%)
90
85
80
75
70
0.001
V
OUT
= 3.3V
R
SENSE
= 0.033Ω
V
IN
= 5V
EFFICIENCY (%)
I
LOAD
= 1A
85
I
LOAD
= 0.1A
80
75
70
EFFICIENCY (%)
V
IN
= 10V
0.01
0.1
1
LOAD CURRENT (A)
Efficiency vs Input Voltage
V
OUT
= 5V
100
95
V
OUT
= 5V
R
SENSE
= 0.033Ω
I
LOAD
= 1A
0.7
0.6
0.5
SUPPLY CURRENT (µA)
EFFICIENCY (%)
V
IN
– V
OUT
(V)
90
85
I
LOAD
= 0.1A
80
75
70
0
5
20
15
10
INPUT VOLTAGE (V)
Boost Line Regulation
6
5
6
BOOST VOLTAGE (V)
4
3
2
1
0
BOOST VOLTAGE (V)
4
3
2
1
V
SW
= 0V
0
BOOST VOLTAGE (V)
I
BOOST
= 1mA
V
SW
= 0V
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
U W
10
1624 G07
Efficiency vs Input Voltage
V
OUT
= 3.3V
100
95
90
V
OUT
= 3.3V
R
SENSE
= 0.033Ω
100
95
90
85
80
75
Efficiency vs Load Current
V
OUT
= 5V
V
OUT
= 5V
V
IN
= 10V
R
SENSE
= 0.033Ω
0
5
20
15
10
INPUT VOLTAGE (V)
25
30
1624 G09
70
0.001
0.01
0.1
1
LOAD CURRENT (A)
10
1624 G08
V
IN
– V
OUT
Dropout Voltage
vs Load Current
700
R
SENSE
= 0.033Ω
V
OUT
DROP OF 5%
Input Supply Current vs
Input Voltage
V
FB
= 1.21V
600
500
400
300
200
100
SHUTDOWN
0
SLEEP MODE
0.4
0.3
0.2
0.1
0
25
30
1624 G10
0
0.5
1.0
1.5
2.0
LOAD CURRENT (A)
2.5
3.0
1624 G11
0
5
20
15
25
10
INPUT VOLTAGE (V)
30
35
1624 G05
Boost Load Regulation
6.0
V
IN
= 15V
5
V
IN
= 5V
5.5
Boost Voltage vs Temperature
I
LOAD
= 1mA
5.0
4.5
0
5
20
15
25
10
BOOST LOAD CURRENT (mA)
30
1624 G06
4.0
–40 –15
60
35
85
10
TEMPERATURE (°C)
110
135
1624 G04
1624 G15
3
LTC1624
TYPICAL PERFORMANCE CHARACTERISTICS
I
TH
/RUN PIN SOURCE CURRENT WITH V
ITH
= 1V (µA)
V
ITH
vs Output Current
200
2.4
V
ITH
/RUN (V)
I
ITH
(µA)
ACTIVE
MODE
1.2
0.8
SHUTDOWN
0
0
I
OUT
I
OUT(MAX)
(a)
Frequency vs Feedback Voltage
250
250
200
FREQUENCY (kHz)
200
CURRENT SENSE THRESHOLD (mV)
FREQUENCY (kHz)
150
100
50
0
0
0.25
0.75
1.00
0.50
FEEDBACK VOLTAGE
1.25
1624 G03
PIN FUNCTIONS
SENSE
–
(Pin 1):
Connects to the (–) input for the current
comparator. Built-in offsets between the SENSE
–
and V
IN
pins in conjunction with R
SENSE
set the current trip thresh-
olds. Do not pull this pin more than 15V below V
IN
or more
than 0.3V below ground.
I
TH
/RUN (Pin 2):
Combination of Error Amplifier Compen-
sation Point and Run Control Inputs. The current com-
parator threshold increases with this control voltage.
Nominal voltage range for this pin is 1.19V to 2.4V. Forcing
this pin below 0.8V causes the device to be shut down. In
shutdown all functions are disabled and TG pin is held low.
V
FB
(Pin 3):
Receives the feedback voltage from an exter-
nal resistive divider across the output.
GND (Pin 4):
Ground. Connect to the (–) terminal of C
OUT
,
the Schottky diode and the (–) terminal of C
IN
.
SW (Pin 5):
Switch Node Connection to Inductor. In step-
down applications the voltage swing at this pin is from a
Schottky diode (external) voltage drop below ground to
V
IN
.
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1624 G01
I
ITH
vs V
ITH
I
TH
/RUN Pin Source Current vs
Temperature
300
250
200
150
100
50
0
–40 –15
1
I
TH
/RUN = 1V
3
I
TH
/RUN = 0V
2
5
I
TH
/RUN PIN SOURCE CURRENT WITH V
ITH
= 0V (µA)
4
150
50
ACTIVE
MODE
3
SHUTDOWN
0
0
0.8
1.2
V
ITH
(V)
2.4
60
35
85
10
TEMPERATURE (°C)
110
0
135
(b)
1624 G02
1624 G14
Operating Frequency vs
Temperature
170
V
OUT
IN REGULATION
168
166
164
162
160
158
156
154
152
Maximum Current Sense
Threshold vs Temperature
150
100
50
V
FB
= 0V
0
–40 –15
60
85
10
35
TEMPERATURE (°C)
110
135
150
–40 –15
60
35
10
85
TEMPERATURE (°C)
110
135
1448 G12
1448 G13
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LTC1624
PIN FUNCTIONS
TG (Pin 6):
High Current Gate Drive for Top N-Channel
MOSFET. This is the output of a floating driver with a
voltage swing equal to INTV
CC
superimposed on the
switch node voltage SW.
BOOST (Pin 7):
Supply to Topside Floating Driver. The
bootstrap capacitor C
B
is returned to this pin. Voltage
swing at this pin is from INTV
CC
to V
IN
+ INTV
CC
in step-
down applications. In non step-down topologies the volt-
age at this pin is constant and equal to INTV
CC
if SW = 0V.
V
IN
(Pin 8):
Main Supply Pin and the (+) Input to the
Current Comparator. Must be closely decoupled to ground.
OPERATIO
Main Control Loop
The LTC1624 uses a constant frequency, current mode
architecture. During normal operation, the top MOSFET is
turned on each cycle when the oscillator sets the RS latch
and turned off when the main current comparator I
1
resets
the RS latch. The peak inductor current at which I
1
resets
the RS latch is controlled by the voltage on the I
TH
/RUN
pin, which is the output of error amplifier EA. The V
FB
pin,
described in the pin functions, allows EA to receive an
output feedback voltage from an external resistive divider.
When the load current increases, it causes a slight
decrease in V
FB
relative to the 1.19V reference, which in
turn causes the I
TH
/RUN voltage to increase until the
average inductor current matches the new load current.
While the top MOSFET is off, the internal bottom MOSFET
is turned on for approximately 300ns to 400ns to recharge
the bootstrap capacitor C
B
.
The top MOSFET driver is biased from the floating boot-
strap capacitor C
B
that is recharged during each off cycle.
The dropout detector counts the number of oscillator
cycles that the top MOSFET remains on and periodically
forces a brief off period to allow C
B
to recharge.
The main control loop is shut down by pulling the I
TH
/RUN
pin below its 1.19V clamp voltage. Releasing I
TH
/RUN
allows an internal 2.5µA current source to charge com-
pensation capacitor C
C
. When the I
TH
/RUN pin voltage
reaches 0.8V the main control loop is enabled with the I
TH
/
RUN voltage pulled up by the error amp. Soft start can be
U
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U
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(Refer to Functional Diagram)
implemented by ramping the voltage on the I
TH
/RUN pin
from 1.19V to its 2.4V maximum (see Applications Infor-
mation section).
Comparator OV guards against transient output over-
shoots >7.5% by turning off the top MOSFET and keeping
it off until the fault is removed.
Low Current Operation
The LTC1624 is capable of Burst Mode operation in which
the external MOSFET operates intermittently based on
load demand. The transition to low current operation
begins when comparator B detects when the I
TH
/RUN
voltage is below 1.5V. If the voltage across R
SENSE
does
not exceed the offset of I
2
(approximately 20mV) for one
full cycle, then on following cycles the top and internal
bottom drives are disabled. This continues until the I
TH
voltage exceeds 1.5V, which causes drive to be returned to
the TG pin on the next cycle.
INTV
CC
Power/Boost Supply
Power for the top and internal bottom MOSFET drivers is
derived from V
IN
. An internal regulator supplies INTV
CC
power. To power the top driver in step-down applications
an internal high voltage diode recharges the bootstrap
capacitor C
B
during each off cycle from the INTV
CC
supply.
A small internal N-channel MOSFET pulls the switch node
(SW) to ground each cycle after the top MOSFET has
turned off ensuring the bootstrap capacitor is kept fully
charged.
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