LTC1649
3.3V Input High Power
Step-Down Switching
Regulator Controller
FEATURES
s
s
s
s
DESCRIPTIO
s
s
s
s
High Power 3.3V to 1.xV-2.xV Switching Regulator
Controller: Up to 20A Output
All N-Channel External MOSFETs
Provides 5V MOSFET Gate Drive with 3.3V Input
Excellent Output Regulation:
±1%
Over Line, Load
and Temperature Variations
Constant Frequency Operation Minimizes
Inductor Size
High Efficiency: Over 90% Possible
No Low-Value Sense Resistor Needed
Available in 16-Lead SO Package
The LTC
®
1649 is a high power, high efficiency switching
regulator controller optimized for use with very low supply
voltages. It operates from 2.7V to 5V input, and provides
a regulated output voltage from 1.26V to 2.5V at up to 20A
load current. A typical 3.3V to 2.5V application features
efficiency above 90% from 1A to 10A load. The LTC1649
uses a pair of standard 5V logic-level N-channel external
MOSFETs, eliminating the need for expensive P-channel
or super-low-threshold devices.
The LTC1649 shares its internal switching architecture
with the LTC1430, and features the same
±1%
line, load
and temperature regulation characteristics. Current limit
is user-adjustable without requiring an external low-value
sense resistor. The LTC1649 uses a 200kHz switching
frequency and voltage mode control, minimizing external
component count and size. Shutdown mode drops the
quiescent current to below 10µA.
The LTC1649 is available in the 16-pin narrow SO package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
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3.3V Input Power Supply for Low Voltage
Microprocessors and Logic
Low Input Voltage Power Supplies
High Power, Low Voltage Regulators
Local Regulation for Multiple Voltage Distributed
Power Systems
TYPICAL APPLICATIO
V
IN
3.3V
MBR0530
R
IMAX
50k
3.3V to 2.5V, 15A Converter
Q1, Q2
IRF7801
TWO IN
PARALLEL
+
1µF
C
IN
3300µF
L
EXT
*
1.2µH
V
OUT
2.5V
@15A
P
VCC1
22Ω
P
VCC2
G1
1k
I
FB
Q3
IRF7801
R1
12.4k
EFFICIENCY (%)
G2
V
CC
LTC1649
FB
I
MAX
SHDN
10µF
R
C
7.5k
C
C
0.01µF
C1
220pF
0.1µF
+
SHDN
COMP
SS
GND
V
IN
C
+
1µF
C
–
CP
OUT
R2
12.7k
+
C
OUT
4400µF
+
MBR0530
10µF
0.33µF
IRF7801 = INTERNATIONAL RECTIFIER
MBR0530 = MOTOROLA
*12TS-1R2HL = PANASONIC
1649 TA01
U
LTC1649 Efficiency
100
90
80
70
60
50
40
0.1
1
LOAD CURRENT (A)
10
1649 TA02
U
U
1
LTC1649
ABSOLUTE
MAXIMUM
RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW
G1 1
P
VCC1
2
GND 3
FB 4
SHDN 5
SS 6
V
IN
7
C
–
8
16 G2
15 P
VCC2
14 V
CC
13 I
FB
12 I
MAX
11 COMP
10 CP
OUT
9
C
+
Supply Voltage
V
IN ...........................................................................................
6V
V
CC ...........................................................................................
9V
P
VCC1, 2 ................................................................................
13V
Input Voltage
I
FB .......................................................................
– 0.3V to 18V
C
+
, C
– ................................................
– 0.3V to (V
IN
+ 0.3V)
All Other Inputs ....................... – 0.3V to (V
CC
+ 0.3V)
Operating Temperature Range
LTC1649C ............................................... 0°C to 70°C
LTC1649I ............................................ – 40°C to 85°C
Storage Temperature Range ................. – 65°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
ORDER PART
NUMBER
LTC1649CS
LTC1649IS
S PACKAGE
16-LEAD PLASTIC SO
T
JMAX
= 150°C,
θ
JA
= 110°C/ W
Consult factory for parts specified with wider operating temperature ranges.
The
q
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 3.3V (Note 2)
SYMBOL PARAMETER
V
IN
V
FB
V
CPOUT
I
IN
Minimum Supply Voltage
Feedback Voltage
Charge Pump Output Voltage
Supply Current (V
IN
)
CONDITIONS
Figure 1 (Note 3)
Figure 1
Figure 1
V
SHDN
= V
CC
, I
LOAD
= 0
V
SHDN
= 0V
P
VCC
= 5V, V
SHDN
= V
CC
(Note 4)
V
SHDN
= 0V
I
CPOUT
= 20mA (Note 5)
q
q
q
q
q
q
q
q
ELECTRICAL CHARACTERISTICS
LTC1649CS
MIN
TYP
MAX
2.7
1.25
4.8
1.265
5
3
10
1.5
0.1
700
140
2.4
0.8
±0.01
650
1300
q
q
LTC1649IS
MIN
TYP
MAX
2.7
1.23
4.75
1.265
5
3
10
1.5
0.1
700
1.29
5.25
5
25
UNITS
V
V
V
mA
µA
mA
µA
kHz
1.28
5.2
5
25
I
PVCC1, 2
Supply Current (P
VCC1, 2
)
f
CP
f
OSC
V
IH
V
IL
I
IN
gm
V
gm
I
I
IMAX
I
SS
t
r
, t
f
t
NOV
DC
MAX
Internal Charge Pump Frequency
Internal PWM Oscillator Frequency
SHDN Input High Voltage
SHDN Input Low Voltage
SHDN Input Current
Error Amplifier Transconductance
I
LIM
Amplifier Transconductance
I
MAX
Sink Current
Soft Start Source Current
Driver Rise/Fall Time
Driver Non-Overlap Time
Maximum Duty Cycle
200
260
130
2.4
200
325
0.8
±1
±0.01
650
1300
±1
µMho
µMho
17
–17
250
250
µA
µA
ns
ns
%
(Note 6)
V
IMAX
= V
CC
V
SS
= 0V
P
VCC1
= P
VCC2
= 5V (Note 7)
P
VCC1
= P
VCC2
= 5V
V
COMP
= V
CC
25
90.5
8
–8
12
–12
80
130
93
16
–16
250
250
8
–8
25
90.5
12
–12
80
130
93
2
U
W
U
U
W W
W
kHz
V
V
µA
LTC1649
ELECTRICAL CHARACTERISTICS
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a part may be impaired.
Note 2:
All currents into device pins are positive; all currents out of device
pins are negative. All voltages are referenced to ground unless otherwise
specified.
Note 3:
Maximum Duty Cycle limitations will limit the output voltage
obtainable at very low supply voltages.
Note 4:
Supply current at P
VCC1
and P
VCC2
is dominated by the current
needed to charge and discharge the external MOSFET gates. This current
will vary with the operating voltage and the external MOSFETs used.
Note 5:
Under normal operating conditions, the charge pump will skip
cycles to maintain regulation and the apparent frequency will be lower than
700kHz.
Note 6:
The I
LIM
amplifier can sink but not source current. Under normal
(not current limited) operation, the I
LIM
output current will be zero.
Note 7:
Driver rise and fall times are measured from 10% to 90%.
TYPICAL PERFOR A CE CHARACTERISTICS
I
MAX
Pin Current vs Temperature
14.0
13.5
V
CC
= 5V
240
V
CC
= 5V
230
220
210
200
190
180
170
– 40 –20
I
MAX
CURRENT (µA)
12.5
12.0
11.5
11.0
10.5
– 40 –20
DUTY CYCLE (%)
13.0
OSCILLATOR FREQUENCY (kHz)
40
20
60
0
TEMPERATURE (°C)
Error Amplifier Transconductance
vs Temperature
850
800
∆I
g
m
=
COMP
∆V
FB
V
CC
= 5V
∆V
OUT
(mV)
TRANSCONDUCTANCE (µmho)
750
700
650
600
550
500
450
400
350
– 40
– 20
0
– 0.2
– 0.4
– 0.6
– 0.8
–1.0
0
OUTPUT VOLTAGE (V)
20
0
60
40
TEMPERATURE (°C)
U W
80
1649 G01
Oscillator Frequency
vs Temperature
100
95
90
85
80
75
Maximum Duty Cycle
vs Temperature
V
COMP
= V
CC
V
FB
= 1.265V
V
CC
= 5V
100
40
20
60
0
TEMPERATURE (°C)
80
100
70
– 40
– 20
20
0
60
40
TEMPERATURE (°C)
80
100
1649 G02
1649 G03
Load Regulation
0.4
0.2
T
A
= 25°C
V
OUT
= 3.3V
V
CC
= 5V
FIGURE 1
4.0
3.5
3.0
2.5
2.0
Output Voltage vs Load Current
with Current Limit
R
IMAX
= 16k
1.5
1.0
0.5
0
T
A
= 25°C
V
CC
= 5V
FIGURE 1
0
2
R
IMAX
= 33k
80
100
1
2
3 4 5 6 7
LOAD CURRENT (A)
8
9
10
8
6
LOAD CURRENT (A)
4
10
12
1649 G07
1649 G04
1649 G06
3
LTC1649
PIN FUNCTIONS
G1 (Pin 1):
Driver Output 1. Connect this pin to the gate of
the upper N-channel MOSFET, Q1. This output will swing
from P
VCC1
to GND. G1 will always be low when G2 is high.
In shutdown, G1 and G2 go low.
P
VCC1
(Pin 2):
Power V
CC
for Driver 1. This is the power
supply input for G1. G1 will swing from P
VCC1
to GND.
P
VCC1
must be connected to a potential of at least V
IN
+
V
GS(ON)
(Q1). This potential can be generated using a
simple charge pump connected to the switching node
between the two external MOSFETs as shown in Figure 1.
GND (Pin 3):
System Ground. Connect to a low impedance
ground in close proximity to the source of Q2. The system
signal and power grounds should meet at only one point,
at the GND pin of the LTC1649.
FB (Pin 4):
Feedback. The FB pin is connected to the output
through a resistor divider to set the output voltage.
V
OUT
= V
REF
[1 + (R1/R2)].
SHDN (Pin 5):
Shutdown, Active Low. A TTL compatible
LOW level at SHDN for more than 50µs puts the LTC1649
into shutdown mode. In shutdown, G1, G2, COMP and SS
go low, and the quiescent current drops to 25µA max.
CP
OUT
remains at 5V in shutdown mode. A TTL compatible
HIGH level at SHDN allows the LTC1649 to operate nor-
mally.
SS (Pin 6):
Soft Start. An external capacitor from SS to
GND controls the startup time and also compensates the
current limit loop, allowing the LTC1649 to enter and exit
current limit cleanly.
V
IN
(Pin 7):
Charge Pump Input. This is the main low
voltage power supply input. V
IN
requires an input voltage
between 3V and 5V. Bypass V
IN
to ground with a 1µF
ceramic capacitor located close to the LTC1649.
C
–
(Pin 8):
Flying Capacitor, Negative Terminal. Connect
a 1µF ceramic capacitor from C
–
to C
+
.
C
+
(Pin 9):
Flying Capacitor, Positive Terminal.
CP
OUT
(Pin 10):
Charge Pump Output. CP
OUT
provides a
regulated 5V output to provide power for the internal
switching circuitry and gate drive for the external MOSFETs.
CP
OUT
should be connected directly to P
VCC2
in most
applications. At least 10µF of reservoir capacitance to
ground is required at CP
OUT
. This requirement can usually
be met by the bypass capacitor at P
VCC2
.
COMP (Pin 11):
External Compensation. The COMP pin is
connected directly to the output of the internal error
amplifier and the input of the PWM generator. An RC
network is used at this node to compensate the feedback
loop to provide optimum transient response.
I
MAX
(Pin 12):
Current Limit Set. I
MAX
sets the threshold
for the internal current limit comparator. If I
FB
drops below
I
MAX
with G1 on, the LTC1649 will go into current limit.
I
MAX
has an internal 12µA pull-down to GND. The voltage
at I
MAX
can be set with an external resistor to the drain of
Q1 or with an external voltage source.
I
FB
(Pin 13):
Current Limit Sense. Connect to the switched
node at the source of Q1 and the drain of Q2 through a 1kΩ
resistor. The resistor is required to prevent voltage tran-
sients at the switched node from damaging the I
FB
pin. I
FB
can be taken up to 18V above GND without damage.
V
CC
(Pin 14):
Internal Power Supply. V
CC
provides power
to the feedback amplifier and switching control circuits.
V
CC
is designed to run from the 5V supply provided by
CP
OUT
. V
CC
requires a 10µF bypass capacitor to GND.
P
VCC2
(Pin 15):
Power V
CC
for Driver 2. This is the power
supply input for G2. G2 will swing from P
VCC2
to GND.
P
VCC2
must be connected to a potential of at least
V
GS(ON)
(Q2). This voltage is usually supplied by the CP
OUT
pin. P
VCC2
requires a bypass capacitor to GND; this
capacitor also provides the reservoir capacitance required
by the CP
OUT
pin.
G2 (Pin 16):
Driver Output 2. Connect this pin to the gate
of the lower N-channel MOSFET, Q2. This output will
swing from P
VCC2
to GND. G2 will always be low when G1
is high. In shutdown, G1 and G2 go low.
4
U
U
U
LTC1649
BLOCK DIAGRA
V
IN
SHDN
COMP
V
CC
12µA
SS
I
LIM
FB
MIN
MAX
I
MAX
12µA
40mV
+
1.26V
1649 BD
TEST CIRCUIT
V
IN
3.3V
MBR0530
R
IMAX
50k
1µF
Q1, Q2
IRF7801
TWO IN
PARALLEL
P
VCC1
22Ω
P
VCC2
G1
1k
I
FB
Q3
IRF7801
R1
12.4k
G2
V
CC
LTC1649
FB
I
MAX
SHDN
10µF
R
C
7.5k
C
C
0.01µF
C1
220pF
0.1µF
+
SHDN
COMP
SS
GND
V
IN
C
+
1µF
C
–
CP
OUT
R2
12.7k
+
MBR0530
Figure 1
+
W
DELAY
50µs
INTERNAL
SHUTDOWN
PWM
C
+
C
–
CHARGE
PUMP
CP
OUT
PV
CC1
G1
PV
CC2
G2
–
+
+
–
I
FB
+
40mV
FB
+
C
IN
3300µF
L
EXT
1.2µH
V
OUT
2.5V
+
C
OUT
4400µF
10µF
0.33µF
1649 TA03
5
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