Linear Technology LTC1649 Datasheet

FEATURES

■

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

■

Constant Frequency Operation Minimizes
Inductor Size

■

Excellent Output Regulation: ±1% Over Line, Load
and Temperature Variations

■

High Efficiency: Over 90% Possible

■

No Low-Value Sense Resistor Needed

■

Available in 16-Lead SO Package

U

APPLICATIONS

■

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

LTC1649

3.3V Input High Power
Step-Down Switching

Regulator Controller

U

DESCRIPTION

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.

TYPICAL APPLICATION

3.3V to 2.5V, 15A Converter LTC1649 Efficiency

V

IN

3.3V

MBR0530

22Ω 1k

SHDN

+

10µF

R

7.5k

C1
220pF

IRF7801 = INTERNATIONAL RECTIFIER
MBR0530 = MOTOROLA
*12TS-1R2HL = PANASONIC

C

C

C

0.01µF

R

IMAX

50k

0.1µF

P

VCC1

P

VCC2

V

CC

LTC1649

I

MAX

SHDN

COMP
SS C
GND CP

MBR0530

OUT

G1

I

FB

G2

FB

V

IN

+

C

–

U

+

1µF

10µF

1µF

0.33µF

Q1, Q2
IRF7801
TWO IN
PARALLEL

Q3
IRF7801

+

C

IN

3300µF

*

L

EXT

1.2µH

V

OUT

2.5V

12.4k

12.7k

R1

R2

@15A

+

C

OUT

4400µF

1649 TA01

100

90

80

70

EFFICIENCY (%)

60

50

40

0.1 1 10
LOAD CURRENT (A)

1649 TA02

1

LTC1649

WW

W

ABSOLUTE MAXIMUM RATINGS

U

UUW

PACKAGE/ORDER INFORMATION

(Note 1)

Supply Voltage

V

...........................................................................................

IN

V

...........................................................................................

CC

P

VCC1, 2

................................................................................

6V
9V

13V

Input Voltage

I

.......................................................................

FB

–

C+, C

................................................

–0.3V to (VIN + 0.3V)

–0.3V to 18V

All Other Inputs ....................... –0.3V to (VCC + 0.3V)

Operating Temperature Range ..................... 0°C to 70°C

Storage Temperature Range ................. –65°C to 150°C

Lead Temperature (Soldering, 10 sec)..................300°C

P

VCC1

GND

SHDN

V

G1

FB

SS

C

TOP VIEW

1
2
3
4
5
6
7

IN

–

8

S PACKAGE

16-LEAD PLASTIC SO

T

= 150°C, θJA = 110°C/W

JMAX

16

G2

15

P

VCC2

14

V

CC

13

I

FB

12

I

MAX

11

COMP

10

CP

OUT

+

9

C

Consult factory for Industrial and Military grade parts.

ELECTRICAL CHARACTERISTICS

VIN = 3.3V, TA = 25°C unless otherwise noted. (Note 2)

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V

IN

V

FB

V

CPOUT

I

IN

I

PVCC1, 2

f

CP

f

OSC

V

IH

V

IL

I

IN

gm

V

gm

I

I

IMAX

I

SS

tr, t

f

t

NOV

DC

MAX

The ● denotes specifications which apply over the full operating
temperature range.

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

Minimum Supply Voltage Figure 1 (Note 3) ● 2.7 V
Feedback Voltage Figure 1 ● 1.25 1.265 1.28 V
Charge Pump Output Voltage Figure 1 ● 4.8 5 5.2 V
Supply Current (VIN)V

Supply Current (P

)P

VCC1, 2

Internal Charge Pump Frequency I

= VCC, I

SHDN

= 0V 10 25 µA

V

SHDN

= 5V, V

VCC

= 0V 0.1 µA

V

SHDN

= 20mA (Note 5) 700 kHz

CPOUT

= 0 ● 35 mA

LOAD

= VCC (Note 4) 1.5 mA

SHDN

Internal PWM Oscillator Frequency ● 140 200 260 kHz
SHDN Input High Voltage ● 2.4 V
SHDN Input Low Voltage ● 0.8 V
SHDN Input Current ● ±0.01 ±1 µA
Error Amplifier Transconductance 650 µMho
I

Amplifier Transconductance (Note 6) 1300 µMho

LIM

I

Sink Current V

MAX

= VCC ● 81216 µA

IMAX

Soft Start Source Current VSS = 0V ● –8 –12 –16 µA
Driver Rise/Fall Time P
Driver Non-Overlap Time P
Maximum Duty Cycle V

VCC1

VCC1

COMP

= P

= 5V 80 250 ns

VCC2

= P

= 5V 25 130 250 ns

VCC2

= V

CC

Note 4: Supply current at P

VCC1

90.5 93 %

and P

VCC2

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
(not current limited) operation, the I

amplifier can sink but not source current. Under normal

LIM

output current will be zero.

LIM

obtainable at very low supply voltages.

ORDER PART

NUMBER

LTC1649CS

is dominated by the current

2

UW

TYPICAL PERFOR A CE CHARACTERISTICS

I

Pin Current vs Temperature

MAX

14.0
VCC = 5V

13.5

LTC1649

Oscillator Frequency
vs Temperature

240

VCC = 5V

230

13.0

12.5

12.0

CURRENT (µA)

MAX

I

11.5

11.0

10.5

–40

–20 0

Maximum Duty Cycle
vs Temperature

100

V

= V

COMP

VFB = 1.265V

95

V

= 5V

CC

90

85

80

DUTY CYCLE (%)

75

70

–20 20

–40

0

40 80 100

20 60

TEMPERATURE (°C)

CC

TEMPERATURE (°C)

40

220

210

200

190

OSCILLATOR FREQUENCY (kHz)

180

1649 G01

170

–40

–20 0

TEMPERATURE (°C)

40 80 100

20 60

1649 G02

Error Amplifier Transconductance
vs Temperature

850

∆I

800
750
700
650
600
550
500
450

TRANSCONDUCTANCE (µmho)

400

80

1649 G03

100

60

350

–40

–20 20

0

TEMPERATURE (°C)

40

COMP

gm =

∆V

FB

VCC = 5V

60 100

80

1649 G04

Load Regulation

0.4
TA = 25°C
V

OUT

0.2

–0.2

(mV)

OUT

–0.4

∆V

–0.6

–0.8

–1.0

0

V
FIGURE 1

0

= 5V

CC

1

= 3.3V

3

4

2

LOAD CURRENT (A)

7

68

5

9

1649 G06

Output Voltage vs Load Current
with Current Limit

4.0

3.5

3.0

2.5

2.0

1.5

OUTPUT VOLTAGE (V)

1.0

0.5

10

R

= 16k R

IMAX

TA = 25°C

= 5V

V

CC

FIGURE 1

0

24 8

0

LOAD CURRENT (A)

= 33k

IMAX

12

6

10

1649 G07

3

LTC1649

UUU

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

to GND. G1 will always be low when G2 is high.

VCC1

In shutdown, G1 and G2 go low.

P

(Pin 2): Power VCC for Driver 1. This is the power

VCC1

supply input for G1. G1 will swing from P
P

must be connected to a potential of at least VIN +

VCC1

V

(Q1). This potential can be generated using a

GS(ON)

VCC1

to GND.

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

[1 + (R1/R2)].

REF

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

remains at 5V in shutdown mode. A TTL compatible

OUT

HIGH level at SHDN allows the LTC1649 to operate nor­mally.

C+ (Pin 9): Flying Capacitor, Positive Terminal.
CP

(Pin 10): Charge Pump Output. CP

OUT

provides a

OUT

regulated 5V output to provide power for the internal
switching circuitry and gate drive for the external MOSFETs.
CP

should be connected directly to P

OUT

VCC2

in most

applications. At least 10µF of reservoir capacitance to
ground is required at CP
be met by the bypass capacitor at P

. This requirement can usually

OUT

.

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

(Pin 12): Current Limit Set. I

MAX

sets the threshold

MAX

for the internal current limit comparator. If IFB drops below
I

with G1 on, the LTC1649 will go into current limit.

MAX

I

has an internal 12µA pull-down to GND. The voltage

MAX

at I

can be set with an external resistor to the drain of

MAX

Q1 or with an external voltage source.
IFB (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 tran­sients at the switched node from damaging the IFB pin. I

FB

can be taken up to 18V above GND without damage.

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.

VIN (Pin 7): Charge Pump Input. This is the main low
voltage power supply input. VIN requires an input voltage
between 3V and 5V. Bypass VIN 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+.

4

VCC (Pin 14): Internal Power Supply. VCC provides power
to the feedback amplifier and switching control circuits.
VCC is designed to run from the 5V supply provided by
CP

. VCC requires a 10µF bypass capacitor to GND.

OUT

P

(Pin 15): Power VCC for Driver 2. This is the power

VCC2

supply input for G2. G2 will swing from P
P

must be connected to a potential of at least

VCC2

V
pin. P

(Q2). This voltage is usually supplied by the CP

GS(ON)

requires a bypass capacitor to GND; this

VCC2

VCC2

to GND.

OUT

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

to GND. G2 will always be low when G1

VCC2

is high. In shutdown, G1 and G2 go low.

LTC1649

BLOCK DIAGRA

V

IN

SHDN

COMP

V

CC

12µA

SS

I

MAX

12µA

W

DELAY

50µs

I

LIM

+

–

+

INTERNAL
SHUTDOWN

PWM

FB MIN

–

40mV

+

+

C

CHARGE

PUMP

C

+

–

MAX

40mV

CP

OUT

PV

CC1

G1

PV

CC2

G2

I

FB

FB

TEST CIRCUIT

V

IN

3.3V

SHDN

+

10µF

+

1.26V

R

IMAX

MBR0530

22Ω 1k

R

C

7.5k
C

C

0.01µF

C1
220pF

50k

0.1µF

P

VCC1

P

VCC2

V

CC

LTC1649

I

MAX

SHDN

COMP
SS C
GND CP

MBR0530

G1

I

G2

V

OUT

FB

FB

IN

+

C

–

1µF

1µF

+

10µF

0.33µF

Q1, Q2
IRF7801
TWO IN
PARALLEL

Q3
IRF7801

1649 BD

+

C

IN

3300µF

L

EXT

1.2µH

V

OUT

2.5V

R1

12.4k

+

C

OUT

4400µF

R2

12.7k

Figure 1

1649 TA03

5