Datasheet LTC1147L, LTC1147-5, LTC1147-3.3 Datasheet (Linear Technology)

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FEATURES

■

Very High Efficiency: Over 95% Possible

■

Wide VIN Range: 3.5V* to 16V

■

Current Mode Operation for Excellent Line and Load Transient Response

■

High Efficiency Maintained Over Three Decades of Output Current

■

Low 160µA Standby Current at Light Loads

■

Logic Controlled Micropower Shutdown: IQ < 20µA

■

Short-Circuit Protection

■

Very Low Dropout Operation: 100% Duty Cycle

■

High Efficiency in a Small Amount of Board Space

■

Output Can Be Externally Held High in Shutdown

■

Available in 8-Pin SO Package

LTC1147-3.3

LTC1147-5/LTC1147L

High Efficiency Step-Down

Switching Regulator Controllers

DESCRIPTIO

The LTC®1147 series are step-down switching regulator controllers featuring automatic Burst ModeTM operation to maintain high efficiencies at low output currents. These devices drive an external P-channel power MOSFET at switching frequencies exceeding 400kHz using a constant off-time current mode architecture providing constant ripple current in the inductor.

The operating current level is user-programmable via an external current sense resistor. Wide input supply range allows operation from 3.5V* to 14V (16V maximum). Constant off-time architecture provides low dropout regulation limited by only the R and resistance of the inductor and current sense resistor.

of the external MOSFET

DS(ON)

APPLICATIO S

■

Notebook and Palmtop Computers

■

Portable Instruments

■

Battery-Operated Digital Devices

■

Cellular Telephones

■

DC Power Distribution Systems

■

GPS Systems

PPLICATITYPICAL

(5.2V TO 14V)

1µF

0V = NORMAL

>1.5V = SHUTDOWN

RC 1k

CC 3300pF

CT 470pF

Figure 1. High Efficiency Step-Down Converter

SHDN

PDRIVE

LTC1147-5

SENSE SENSE

GND

–

P-CHANNEL Si4431DY

1000pF

D1 MBRD330

The LTC1147 series incorporates automatic power saving Burst Mode operation to reduce switching losses when load currents drop below the level required for continuous operation. Standby power is reduced to only 2mW at VIN = 10V (at I operation are typically 0mA to 300mA.

For applications where even higher efficiency is required, refer to the LTC1148 data sheet and Application Note 54.

Burst Mode is a trademark of Linear Technology Corporation. *LTC1147L and LTC1147L-3.3 only.

CIN 100µF

L*

**

50µH

*

COILTRONICS CTX50-2-MP

KRL SL-1-C1-0R050J

SENSE

0.05Ω

= 0). Load currents in Burst Mode

OUT

, LTC and LT are registered trademarks of Linear Technology Corporation.

LTC1147-5 Efficiency

100

VIN = 6V

OUT

5V/2A

OUT

390µF

LT1147 • F01



EFFICIENCY (%)

0.001

VIN = 10V

0.01 0.1 1 LOAD CURRENT (A)

LT1147 • TA01

LTC1147-3.3 LTC1147-5/LTC1147L

ABSOLUTE AXI U RATI GS

Input Supply Voltage (Pin 1)..................... 16V to –0.3V

Continuous Output Current (Pin 8)...................... 50mA

Sense Voltages (Pins 4, 5)

VIN ≥ 12.7V ...........................................13V to –0.3V

VIN < 12.7V............................... (VIN + 0.3V) to –0.3V

UUW

PACKAGE/ORDER I FOR A TIO

TOP VIEW



1



2



3

–

SENSE

N8 PACKAGE

8-LEAD PLASTIC DIP

* ADJUSTABLE OUTPUT VERSION

= 125°C, θ

JMAX

= 125°C, θ

JMAX

8-LEAD PLASTIC SO

= 110°C/W (N)

= 150°C/W (S)

PDRIVE

8

GND

7

SHDN 

6

SENSE

5



S8 PACKAGE

LTC1147CN8-3.3 LTC1147CN8-5

LTC1147CS8-3.3 LTC1147CS8-5 LTC1147IS8-3.3

Operating Ambient Temperature Range

LTC1147C................................................0°C to 70°C

LTC1147I.............................................–40°C to 85°C

Extended Commercial

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

ORDER PART NUMBER S8 PART MARKING

LTC1147LCS8 LTC1147LCS8-3.3 LTC1147LIS8

11473 11475 1147L 1147L3 1147LI 1147I3

Consult factory for Military grade parts.

ELECTRICAL CHARACTERISTICS

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V I V

∆V

OUT

Feedback Voltage (LTC1147L) VIN = 9V ● 1.21 1.25 1.29 V Feedback Current (LTC1147L) ● 0.2 1 µA Regulated Output Voltage VIN = 9V

LTC1147-3.3, LTC1147L-3.3 I

LTC1147-5 I Output Voltage Line Regulation VIN = 7V to 12V, I Output Voltage Load Regulation

LTC1147-3.3, LTC1147L-3.3 5mA < I

LTC1147-5 5mA < I Burst Mode Output Ripple I Input DC Supply Current (Note 2) (Note 5)

LTC1147 Series

Normal Mode 4V < VIN < 12V 1.6 2.1 mA Sleep Mode 4V < V Sleep Mode (LTC1147-5) 5V < V Shutdown V

LTC1147L Series

Normal Mode 3.5V < VIN < 12V 1.6 2.1 mA Sleep Mode 3.5V < V Shutdown (LTC1147L-3.3) V

TA = 25°C, VIN = 10V, V

= 700mA ● 3.23 3.33 3.43 V

LOAD

= 700mA ● 4.90 5.05 5.20 V

LOAD

= 50mA –40 0 40 mV

LOAD

< 2A ● 40 65 mV

LOAD

< 2A ● 60 100 mV

LOAD

= 0A 50 mV

LOAD

< 12V 160 230 µA

= 2.1V, 4V < VIN < 12V 10 20 µA

SHDN

< 12V 160 230 µA

= 2.1V, 3.5V < VIN < 12V 10 20 µA

SHDN

= 0V, unless otherwise noted.

SHDN

P-P

LTC1147-3.3

LTC1147-5/LTC1147L

ELECTRICAL CHARACTERISTICS

TA = 25°C, VIN = 10V, V

= 0V, unless otherwise noted.

SHDN

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

V5 – V

Current Sense Threshold Voltage (Note 6)

LTC1147-3.3, LTC1147L-3.3 V

LTC1147–5 V

LTC1147L V

SHDN Pin Threshold

SENSE

SENSE SENSE

SENSE

–

= V

+ 100mV (Forced) 25 mV

OUT

–

= V

– 100mV (Forced) 130 150 170 mV

OUT

–

= V

+ 100mV (Forced) 25 mV

OUT

–

= V

– 100mV (Forced) 130 150 170 mV

OUT

–

= 5V, V6 = V

–

= 5V, V6 = V

/4 + 25mV (Forced) 25 mV

OUT

/4 – 25mV (Forced) 130 150 170 mV

OUT

LTC1147-3.3/LTC1147-5/LTC1147L-3.3 0.5 0.8 2 V

OFF

tr, t

SHDN Pin Input Current

LTC1147-3.3/LTC1147-5/LTC1147L-3.3 0V < V CT Pin Discharge Current V

OUT

Off-Time (Note 3) CT = 390pF, I

< 8V, VIN = 16V 1.2 5 µA

SHDN

in Regulation, V

SENSE

–

= V

OUT

50 70 90 µA

= 0V 2 10 µA

= 700mA 4 5 6 µs

LOAD

Driver Output Transition Times CL = 3000pF (Pin 8), VIN = 6V 100 200 ns

–40°C ≤ TA ≤ 85°C (Note 4), VIN = 10V, unless otherwise noted.

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS

OUT

V5 – V

OFF

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

Note 1: T dissipation P

LTC1147CN8-3.3/LTC1147CN8-5: T LTC1147LIS/LTC1147IS8/LTC1147LCS/ LTC1147CS8-3.3/LTC1147CS8-5: T

Note 2: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. See Applications Information.

Feedback Voltage (LTC1147L) VIN = 9V ● 1.20 1.25 1.30 V Regulated Output Voltage VIN = 9V

LTC1147-3.3/LTC1147L-3.3 I

LTC1147-5 I

= 700mA ● 3.17 3.33 3.43 V

LOAD

= 700mA ● 4.85 5.05 5.20 V

LOAD

Input DC Supply Current (Note 2) (Note 5)

LTC1147 Series

Normal Mode 4V < V Sleep Mode 4V < V Sleep Mode (LTC1147-5) 5V < V Shutdown V

< 12V 1.6 2.4 mA

< 12V 160 260 µA

= 2.1V, 4V < VIN < 12V 10 22 µA

SHDN

LTC1147L Series

Normal Mode 3.5V < VIN < 12V 1.6 2.4 mA Sleep Mode 3.5V < V Shutdown (LTC1147L-3.3) V

Current Sense Threshold Voltage (Note 6)

LTC1147-3.3 V

LTC1147-5 V

LTC1147L V

V V V

< 12V 160 260 µA

= 2.1V, 3.5V < VIN < 12V 10 22 µA

SHDN

–

= V

SENSE SENSE SENSE SENSE SENSE SENSE

+ 100mV (Forced) 25 mV

OUT

–

= V

– 100mV (Forced) ● 125 150 185 mV

OUT

–

= V

+ 100mV (Forced) 25 mV

OUT

–

= V

– 100mV (Forced) ● 125 150 185 mV

OUT

–

= 5V, 6V = V

–

= 5V, 6V = V

/4 + 25mV (Forced) 25 mV

OUT

/4 – 25mV (Forced) 125 150 185 mV

OUT

SHDN Pin Threshold

LTC1147-3.3/LTC1147-5/LTC1147L-3.3 0V < V Off-Time (Note 3) CT = 390pF, I

< 8V, VIN = 16V 0.5 0.8 2 V

SHDN

= 700mA 3.8 5 6.5 µs

LOAD

Note 3: In applications where R

is placed at ground potential, the off-

SENSE

time increases approximately 40%.

is calculated from the ambient temperature TA and power

according to the following formulas:

= TA + (PD)(110°C/W)

Note 4: The LTC1147C is guaranteed to meet specified performance from 0°C to 70°C and is designed, characterized and expected to meet these extended temperature limits, but is not tested at –40°C and 85°C. The LTC1147I is guaranteed to meet the extended temperature limits.

= TA + (PD)(150°C/W)

Note 5: The LTC1147L/LTC1147L-3.3 allow operation to V Note 6: The LTC1147L is tested with external feedback resistors resulting

in a nominal output voltage of 2.5V.

= 3.5V.

LTC1147-3.3

LOAD CURRENT (A)

–100

∆V

OUT

(mV)

–80

–60

–40

–20

0.5 1.0 1.5 2.0

LTC1147 • G03

2.5

FIGURE 1 CIRCUIT R

SENSE

= 0.05Ω

VIN = 6V 

VIN = 12V



TEMPERATURE (°C)

SENSE VOLTAGE (mV)

175

125

150

100

MAXIMUM

THRESHOLD

MINIMUM

THRESHOLD

LTC1147 • G09

LTC1147-5/LTC1147L

TYPICAL PERFOR A CE CHARACTERISTICS

Efficiency vs Input Voltage Line Regulation Load Regulation

100

FIGURE 1 CIRCUIT

98 96 94

90 88

EFFICIENCY (%)

86 84 82

LOAD

INPUT VOLTAGE (V)

LOAD

= 100mA

= 1A

LTC1147 • G01

(mV)

OUT

∆V

–10 –20 –30 –40

40 30 20 10

FIGURE 1 CIRCUIT

= 1A

LOAD

INPUT VOLTAGE (V)

812

LTC1147 • G02

2.1

1.8

1.5

1.2

0.9

0.6

SUPPLY CURRENT (mA)

0.3

GATE CHARGE CURRENT (mA)

DC Supply Current Supply Current in Shutdown

NOT INCLUDING GATE CHARGE CURRENT

ACTIVE MODE

SLEEP MODE

8 10 12 14 16 18

INPUT VOLTAGE (V)

Gate Charge Supply Current

QP = 50nC

OPERATING FREQUENCY (kHz)

QP = 29nC

140

200

LTC1147 • G04

260

LTC1147 • G07

(NOT AVAILABLE ON LTC1147L)

16 14 12 10

8 6

SUPPLY CURRENT (µA)

= 2V

SHUTDOWN

INPUT VOLTAGE (V)

Off-Time vs V

80 70 60 50 40 30

OFF-TIME (µs)

20 10

LTC1147-3.3

OUTPUT VOLTAGE (V)

10 18

OUT

SENSE

LTC1147-5

LTC1147 • G05

–

= V

LTC1147 • G08

OUT

Operating Frequency vs (VIN – V

1.6 V

= 5V

OUT

1.4

1.2

1.0

0.8

0.6

0.4

NORMALIZED FREQUENCY

0.2

OUT

(VIN – V

)

) VOLTAGE (V)

OUT

0°C

25°C

70°C 

10 12

LTC1148 • G06

Current Sense Threshold Voltage

UUU

PI FU CTIO S

(Pin 1): Main Supply Pin. Must be closely decoupled

to ground Pin 7. CT (Pin 2): External capacitor CT from Pin 2 to ground sets

the operating frequency. The actual frequency is also dependent upon the input voltage.

ITH (Pin 3): Gain Amplifier Decoupling Point. The current comparator threshold increases with the Pin 3 voltage.

SENSE– (Pin 4): Connects to internal resistive divider which sets the output voltage. Pin 4 is also the (–) input for the current comparator.

SENSE+ (Pin 5): The (+) input to the current comparator. A built-in offset between Pins 4 and 5 in conjunction with R

sets the current trip threshold.

SENSE

LTC1147-3.3

LTC1147-5/LTC1147L

SHDN/VFB (Pin 6): When grounded, the fixed output versions of the LTC1147 family operate normally. Pulling Pin 6 high holds the P-channel MOSFET off and puts the LTC1147 in micropower shutdown mode. Requires CMOS logic signal with tr, tf < 1µ s. Do not leave this pin floating. On the LTC1147L this pin serves as the feedback pin from an external resistive divider used to set the output voltage.

GND (Pin 7): Two independent ground lines must be routed separately to: 1) the (–) terminal of C cathode of the Schottky diode and (–) terminal of CIN.

PDRIVE (Pin 8): High current drive for the P-channel MOSFET. Voltage swing at this pin is from VIN to ground.

, and 2) the

OUT

UUW

FU CTIO AL DIAGRA

SLEEP

–

TH2

TH1

–

Pin 6 Connection Shown For LTC1147-3.3 and LTC1147-5; Changes Create LTC1147L.

PDRIVE

GND

SENSE



–

SENSE

–

R S

OFF-TIME CONTROL

SENSE

–

25mV TO 150mV

–

13k

SHDN

–

REFERENCE

1.25V 100k

LTC1147 • FD

5pF

LTC1147-3.3 LTC1147-5/LTC1147L

OPERATIO

(Refer to Functional Diagram)

The LTC1147 series uses a current mode, constant offtime architecture to switch an external P-channel power MOSFET. Operating frequency is set by an external capacitor at CT (Pin 2).

The output voltage is sensed by an internal voltage divider connected to SENSE– (Pin 4). 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 LTC1147 series automatically switchs between two modes of operation, burst and continuous. The voltage 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.

During the switch “on” cycle in continuous mode, current comparator C monitors the voltage between Pins 4 and 5 connected across an external shunt in series with the inductor. When the voltage across the shunt reaches its threshold value, the PDRIVE output is switched to VIN, turning off the P-channel MOSFET. The timing capacitor connected to Pin 2 is now allowed to discharge at a rate determined by the off-time controller. The discharge current is made proportional to the output voltage (measured by Pin 4) to model the inductor current, which decays at a rate which is also proportional to the output voltage.

When the voltage on the timing capacitor has discharged past V causes the PDRIVE output to go low turning the P-channel MOSFET back on. The cycle then repeats.

As the load current increases, the output voltage decreases slightly. This causes the output of the gain stage

, comparator T trips, setting the flip-flop. This

TH1

(Pin 3) to increase the current comparator threshold, thus tracking the load current.

The sequence of events for Burst Mode operation is very similar to continuous operation with the cycle interrupted by the voltage comparator. When the output voltage is at or above the desired regulated value, the P-channel MOSFET is held off by comparator V and the timing capacitor continues to discharge below V capacitor discharges past V trips, causing the internal sleep line to go low.

The circuit now enters sleep mode with the power MOSFET turned off. In sleep mode, a majority of the circuitry is turned off, dropping the quiescent current from 1.6mA to 160µ A. The load current is now being supplied from the output capacitor. When the output voltage has dropped by the amount of hysteresis in comparator V, the P-channel MOSFET is again turned on and this process repeats.

To avoid the operation of the current loop interfering with Burst Mode operation, a built-in offset VOS is incorporated in the gain stage. This prevents the current comparator threshold from increasing until the output voltage has dropped below a minimum threshold.

Using constant off-time architecture, the operating frequency is a function of the input voltage. To minimize the frequency variation as dropout is approached, the off-time controller increases the discharge current as VIN drops below V turned on continuously (100% duty cycle), providing low dropout operation with V

+ 1.5V. In dropout the P-channel MOSFET is

OUT

TH2

≈ VIN.

. When the timing

TH1

, voltage comparator S

WUU

APPLICATIO S I FOR A TIO

LTC1147L Adjustable Applications

When an output voltage other than 3.3V or 5V is required, the LTC1147L adjustable version is used with an external resistive divider from V The regulated voltage is determined by:

OUT

= 1.25

1 +

)

to VFB (Pin 6) (see Figure 7).

OUT

R2

)

To prevent stray pickup a 100pF capacitor is suggested across R1 located close to the LTC1147L.

For Figure 1 applications with V R inputs operate near ground. When the current comparator is operated at less than 2V common mode, the off-time increases approximately 40%, requiring the use of a smaller timing capacitor CT.

is moved to ground, the current sense comparator

SENSE

below 2V, or when

OUT

LTC1147-3.3

BURST

≈

15mV

SENSE

SC(PK)

150mV R

SENSE

LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR A TIO

The basic LTC1147 application circuit is shown in Figure

1. External component selection is driven by the load requirement and begins with the selection of R R

is known, CT and L can be chosen. Next, the power

SENSE

MOSFET and D1 are selected. Finally, CIN and C selected and the loop is compensated. The circuit shown in Figure 1 can be configured for operation up to an input voltage of 16V. If the application requires higher input voltage, then the synchronous switched LTC1149 should be used. Consult factory for lower minimum input voltage version.

Selection for Output Current

SENSE

is chosen based on the required output current.

SENSE

The LTC1147 series current comparator has a threshold range which extends from a minimum of 25mV/ R

to a maximum of 150mV/R

SENSE

. The current

SENSE

comparator threshold sets the peak of the inductor ripple current, yielding a maximum output current I equal to the peak value less half the peak-to-peak ripple current.

For proper Burst Mode operation, I must be less than or equal to the minimum current comparator threshold.

Since efficiency generally increases with ripple current, the maximum allowable ripple current is assumed, i.e., I

RIPPLE(P-P)

Operating Frequency). Solving for R

= 25mV/R

(see CT and L Selection for

SENSE

and allowing

SENSE

a margin for variations in the LTC1147 series and external component values yields:

SENSE

A graph for selecting R

100mV

MAX

versus maximum output

SENSE

current is given in Figure 2. The load current below in which Burst Mode operation

commences, I I I

, both track I

SC(PK)

and I

BURST

SC(PK)

and the peak short-circuit current

BURST

. Once R

MAX

has been chosen,

SENSE

can be predicted from the following:

. Once

SENSE

are

OUT

MAX

RIPPLE(P-P)

0.20

0.15

(Ω)

0.10

SENSE

0.05

The LTC1147 series automatically extend t

MAXIMUM OUTPUT CURRENT (A)

Figure 2. Selecting R

LTC1147 • F02

SENSE

during a

OFF

short circuit to allow sufficient time for the inductor current to decay between switch cycles. The resulting ripple current causes the average short-circuit current I

SC(AVG)

to be reduced to approximately I

MAX

L and CT Selection for Operating Frequency

The LTC1147 series use a constant off-time architecture with t

determined by an external timing capacitor CT.

OFF

Each time the P-channel MOSFET switch turns on, the voltage on CT is reset to approximately 3.3V. During the off-time, CT is discharged by a current which is proportional to V

. The voltage on CT is analogous to the

OUT

current in inductor L, which likewise decays at a rate proportional to V

. Thus the inductor value must track

OUT

the timing capacitor value. The value of CT is calculated from the desired continuous

mode operating frequency:

CT =

1

(1.3)(10

)(f)

VIN – V

)

+ V

OUT



)

Where VD is the drop across the Schottky diode. A graph for selecting CT versus frequency including the

effects of input voltage is given in Figure 3. As the operating frequency is increased the gate charge

losses will reduce efficiency (see Efficiency Considerations). The complete expression for operating frequency

LTC1147-3.3 LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR ATIO

1000

CAPACITANCE (pF)

is given by:

1

f ≈

OFF

where:

= (1.3)(104)(CT)

OFF

800

600

400

200

VIN = 7V

Figure 3. Timing Capacitor Value

1 –

)

OUT

100

FREQUENCY (kHz)



)

SENSE

VIN = 12V

REG OUT



)

–

200

= V

OUT

VIN = 10V

= 5V

300

LTC1147 • F03

series will delay entering Burst Mode operation and efficiency will be degraded at low currents.

Inductor Core Selection

Once the minimum value for L is known, the type of inductor must be selected. Highest efficiency will be obtained using ferrite, Kool Mµ® (from Magnetics, Inc.) or molypermalloy (MPP) cores. Lower cost powdered iron cores provide suitable performance but cut efficiency by 3% to 5%. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses will increase.

Ferrite designs have very low core loss, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates “hard,” which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple which can cause Burst Mode operation to be falsely triggered in the LTC1147. Do not allow the core to saturate!

is the desired output voltage (i.e., 5V, 3.3V). V

REG

the measured output voltage. Thus V

REG/VOUT

OUT

= 1 in

regulation. Note that as VIN decreases, the frequency decreases.

When the input to output voltage differential drops below 1.5V, the LTC1147 reduces t

by increasing the

OFF

discharge current in CT. This prevents audible operation prior to dropout.

Once the frequency has been set by CT, the inductor L must be chosen to provide no more than 25mV/R

SENSE

of peak-to-peak inductor ripple current. This results in a minimum required inductor value of:

= (5.1)(105)(R

MIN

SENSE

)(CT)(V

REG

)

As the inductor value is increased from the minimum value, the ESR requirements for the output capacitor are eased at the expense of efficiency. If too small an inductor is used, the inductor current will become discontinuous before the LTC1147 series enters Burst Mode operation.

A consequence of this is that the LTC1147

Kool Mµ is a very good, low loss core material for toroids with a “soft” saturation characteristic. Molypermalloy is slightly more efficient at high (>200kHz) switching frequencies but quite a bit more expensive. Toroids are very space efficient, especially when you can use several layers of wire. Because they generally lack a bobbin, mounting is more difficult. However, new designs for surface mount are available from Coiltronics, Sumida and Beckman Industrial Corp. which do not increase the height significantly.

Power MOSFET Selection

An external P-channel power MOSFET must be selected for use with the LTC1147 series. The main selection criteria for the power MOSFET are the threshold voltage V

and “on” resistance R

GS(TH)

DS(ON)

The minimum input voltage determines whether a standard threshold or logic-level threshold MOSFET must be

Kool Mµ is a registered trademark of Magnetics, Inc.

LTC1147-3.3

CIN Required I

RMS

≈ I

MAX

OUT(VIN – VOUT

)]

1/2



LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR A TIO

used. For VIN > 8V, a standard threshold MOSFET (V < 4V) may be used. If VIN is expected to drop below 8V, a logic-level threshold MOSFET (V

GS(TH)

strongly recommended. When a logic-level MOSFET is used, the LTC1147 supply voltage must be less than the absolute maximum VGS ratings for the MOSFET.

The maximum output current I

determines the R

MAX

requirement for the power MOSFET. When the LTC1147 series is operating in continuous mode, the simplifying assumption can be made that either the MOSFET or Schottky diode is always conducting the average load current. The duty cycles for the MOSFET and diode are given by:



P-Ch Duty Cycle =

Schottky Diode Duty Cycle =

From the duty cycle the required R

OUT

DS(ON)

– V

OUT

for the MOSFET

can be derived:

)(PP)

P-Ch R

DS(ON)

OUT

)(I

MAX

)(1 + δP)

GS(TH)

< 2.5V) is

DS(ON)

+ VD)

– V

OUT

+ VD)

LOAD

)ID1 =

Remember to keep lead lengths short and observe proper grounding (see Board Layout Checklist) to avoid ringing and increased dissipation.

The forward voltage drop allowable in the diode is calculated from the maximum short-circuit current as:



VF ≈

SC(PK)

where PD is the allowable power dissipation and will be determined by efficiency and/or thermal requirements (see Efficiency Considerations).

CIN and C

Selection

OUT

In continuous mode, the source current of the P-channel MOSFET is a square wave of duty cycle V

OUT/VIN

. To prevent large voltage transients, a low ESR input capacitor sized for the

maximum RMS current must be used. The

maximum RMS capacitor current is given by:

where PP is the allowable power dissipation and δP is the temperature dependency of R

. PP will be deter-

DS(ON)

mined by efficiency and/or thermal requirements (see Efficiency Considerations). (1 + δ) is generally given for a MOSFET in the form of a normalized R

DS(ON)

vs temperature curve, but δ = 0.007/°C can be used as an approximation for low voltage MOSFETs.

Output Diode Selection (D1)

The Schottky diode D1 shown in Figure 1 only conducts during the off-time. It is important to adequately specify the diode peak current and average power dissipation so as not to exceed the diode ratings.

The most stressful condition for the output diode is under short circuit (V must safely handle I

= 0V). Under this condition the diode

OUT

at close to 100% duty cycle.

SC(PK)

Under normal load conditions the average current conducted by the diode is:

This formula has a maximum at VIN = 2V I

= I

RMS

/2. This simple worst-case condition is com-

OUT

, where

monly used for design because even significant deviations do not offer much relief. Note that capacitor manufacturer’s ripple current ratings are often based on only 2000 hours of life. This makes it advisable to further derate the capacitor, or to choose a capacitor rated at a higher temperature than required. Several capacitors may also be paralleled to meet size or height requirements in the design. Always consult the manufacturer if there is any question. An additional 0.1µ F to 1µ F ceramic decoupling capacitor is also required on VIN (Pin 1) for high frequency decoupling.

The selection of C series resistance (ESR).

than twice the value of R

is driven by the required effective

OUT

The ESR of C

for proper operation of the

SENSE

must be less

OUT

LTC1147:

Required ESR < 2R

OUT

SENSE

LTC1147-3.3 LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR ATIO

Optimum efficiency is obtained by making the ESR equal to R efficiency degrades by less than 1%. If the ESR is greater than 2R will prematurely trigger Burst Mode operation, resulting in disruption of continuous mode and an efficiency hit which can be several percent.

Manufacturers such as Nichicon and United Chemicon should be considered for high performance capacitors. The OS-CON semiconductor dielectric capacitor available from Sanyo has the lowest ESR/size ratio of any aluminum electrolytic at a somewhat higher price. Once the ESR requirement for C rating generally far exceeds the I

In surface mount applications multiple capacitors may have to be paralleled to meet the capacitance, ESR or RMS current handling requirements of the application. Aluminum electrolytic and dry tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalums, available in case heights ranging from 2mm to 4mm. For example, if 200µF/10V is called for in an application requiring 3mm height, two AVX 100µF/10V (P/N TPSD 107K010) could be used. Consult the manufacturer for other specific recommendations.

At low supply voltages, a minimum capacitance at C needed to prevent an abnormal low frequency operating

. As the ESR is increased up to 2R

SENSE

, the voltage ripple on the output capacitor

SENSE

has been met, the RMS current

OUT

RIPPLE(P-P)

1000

800

L = 50µH R

SENSE

= 0.02Ω

, the

SENSE

requirement.

OUT

mode (see Figure 4). When C

is made too small, the

OUT

output ripple at low frequencies will be large enough to trip the voltage comparator. This causes Burst Mode operation to be activated when the LTC1147 series would normally be in continuous operation. The effect is most pronounced with low values of R

and can be im-

SENSE

proved by operating at higher frequencies with lower values of L. The output remains in regulation at all times.

Checking Transient Response

The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, V amount equal to ∆I tive series resistance of C charge or discharge C

(ESR), where ESR is the effec-

LOAD

. ∆I

OUT

until the regulator loop adapts

OUT

LOAD

to the current change and returns V state value. During this recovery time V

shifts by an

OUT

also begins to

to its steady

OUT

can be

OUT

monitored for overshoot or ringing which would indicate a stability problem. The external components shown in the Figure 1 circuit will prove adequate compensation for most applications.

A second, more severe transient is caused by switching in loads with large (>1µ F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with C

, causing a rapid drop in V

OUT

. No regulator

OUT

can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25)C

. Thus a 10µ F capacitor would require a 250µ s

LOAD

rise time, limiting the charging current to about 200mA.

(VIN – V

L = 25µH R

SENSE

) VOLTAGE (V)

OUT

= 0.02Ω

L = 50µH

= 0.05Ω

600

(µF)

OUT

400

200

Figure 4. Minimum Value of C

Efficiency Considerations

The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would

LTC1147 • F04

OUT

produce the most improvement. Percent efficiency can be expressed as:

%Efficiency = 100% – (L1 + L2 + L3 + ...)

LTC1147-3.3

LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR A TIO

where L1, L2, etc., are the individual losses as a percentage of input power. (For high efficiency circuits only small errors are incurred by expressing losses as a percentage of output power.)

Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC1147 circuits: 1) LTC1147 DC bias current,

2) MOSFET gate charge current, 3) I2R losses, and 4) voltage drop of the Schottky diode.

1. The DC supply current is the current which flows into VIN (Pin 1) less the gate charge current. For VIN = 10V the LTC1147 series DC supply current is 160µ A for no load, and increases proportionally with load up to a constant 1.6mA after the LTC1147 series has entered continuous mode. Because the DC bias current is drawn from VIN, the resulting loss increases with input voltage. For VIN = 10V the DC bias losses are generally less than 1% for load currents over 30mA. However, at very low load currents the DC bias current accounts for nearly all of the loss.

2. MOSFET gate charge current results from switching the gate capacitance of the power MOSFET. Each time a MOSFET gate is switched from low to high to low again, a packet of charge dQ moves from VIN to ground. The resulting dQ/dt is a current out of V which is typically much larger than the DC supply current. In continuous mode, I

GATECHG

= f(QP). The typical gate charge for a 0.135Ω P-channel power MOSFET is 40nC. This results in I

GATECHG

= 4mA in 100kHz continuous operation for a 2% to 3% typical midcurrent loss with VIN = 10V.

Note that the gate charge loss increases directly with both input voltage and operating frequency. This is the principal reason why the highest efficiency circuits operate at moderate frequencies. Furthermore, it argues against using a larger MOSFET than necessary to control I2R losses, since overkill can cost efficiency as well as money!

P-channel and Schottky diode. The MOSFET R

DS(ON)

multiplied by the P-channel duty cycle can be summed with the resistances of L and R losses. For example, if R and R at VIN ≈ 2V

= 0.05Ω, then the total

SENSE

. This results in losses ranging from 3%

OUT

= 0.1Ω, RL = 0.15Ω,

DS(ON)

to obtain I2R

SENSE

resistance is 0.3Ω

to 10% as the output current increases from 0.5A to 2A. I2R losses cause the efficiency to roll off at high output currents.

4. The Schottky diode is a major source of power loss at high currents and gets worse at high input voltages. The diode loss is calculated by multiplying the forward voltage drop times the Schottky diode duty cycle multiplied by the load current. For example, assuming a duty cycle of 50% with a Schottky diode forward voltage drop of 0.4V, the loss increases from 0.5% to 8% as the load current increases from 0.5A to 2A.

Figure 5 shows how the efficiency losses in a typical LTC1147 series regulator end up being apportioned. The gate charge loss is responsible for the majority of the efficiency lost in the midcurrent region. If Burst Mode operation was not employed at low currents, the gate charge loss alone would cause efficiency to drop to unacceptable levels. With Burst Mode operation, the DC supply current represents the lone (and unavoidable) loss component which continues to become a higher percentage as output current is reduced. As expected, the I2R losses and Schottky diode loss dominate at high load currents.

100

GATE CHARGE

LTC1147 I

EFFICIENCY/LOSS (%)

SCHOTTKY

I2R

DIODE

3. I2R losses are easily predicted from the DC resistances of the MOSFET, inductor and current shunt. In continuous mode the average output current flows through L and R

, but is “chopped” between the

SENSE

0.03

0.01

Figure 5. Efficiency Loss

0.1

OUTPUT CURRENT (A)

0.3

LTC1147 • F05

LTC1147-3.3

PD = I

SC(AVG)(VD

)

3.3V

LTC1147 • F06



3.3V

LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR ATIO

Other losses including CIN and C losses, MOSFET switching losses, and inductor core losses, generally account for less than 2% total additional loss.

Design Example

As a design example, assume VIN = 5V (nominal), V

3.3V, I

= 1A, and f = 130kHz; R

MAX

immediately be calculated:

R t

= 100mV/1A = 0.1Ω

SENSE

= (1/130kHz)[1 – (3.3/5)] = 2.61µs

OFF

CT = 2.61µs/(1.3)(104) = 220pF L = (5.1)(105)(0.1Ω)(220pF)(3.3V) = 33µH

Assume that the MOSFET dissipation is to be limited to PP = 250mW.

If TA = 50°C and the thermal resistance of the MOSFET is 50°C/W, then the junction temperatures will be 63°C and

δP = 0.007(63 – 25) = 0.27. The required R

MOSFET can now be calculated:

P-Ch R

DS(ON)

5(0.25)

3.3(1)

(1.27)

The P-channel requirement can be met by a Si9430DY. Note that the most stringent requirement for the Schottky diode is with V

= 0 (i.e., short circuit). During a

OUT

continuous short circuit, the worst-case Schottky diode dissipation rises to:

ESR dissipative

OUT

, CT and L can

SENSE

DS(ON)

= 0.3Ω

OUT

for the

MIN

2.61µs

3.3(0.125Ω)(1A)

PP =

1

1 –

)

4.5

3.3

4.5

= 102kHz

)

(1.27)

= 116mW

This last step is necessary to assure that the power dissipation and junction temperature of the P-channel are not exceeded.

Troubleshooting Hints

Since efficiency is critical to LTC1147 series applications, it is very important to verify that the circuit is functioning correctly in both continuous and Burst Mode operation. The waveform to monitor is the voltage on the timing capacitor Pin 2.

In continuous mode (I pin should be a sawtooth with a 0.9V

LOAD

> I

) the voltage on the C

BURST

P-P

swing. This

voltage should never dip below 2V as shown in Figure 6a. When load currents are low (I

LOAD

< I

) Burst Mode

BURST

operation occurs. The voltage on the CT pin now falls to ground for periods of time as shown in Figure 6b. During this time the LTC1147 series are in sleep mode with the quiescent current reduced to 160µA.

The inductor current should also be monitored. Look to verify that the peak-to-peak ripple current in continuous mode operation is approximately the same as in Burst Mode operation.

With the 0.1Ω sense resistor I

SC(AVG)

= 1A will result,

increasing the 0.4V Schottky diode dissipation to 0.4W. CIN will require an RMS current rating of at least 0.5A at

temperature, and C

will require an ESR of 0.1Ω for

OUT

optimum efficiency. Now allow VIN to drop to its minimum value. At lower input

voltages the operating frequency will decrease and the P-channel will be conducting most of the time, causing the power dissipation to increase. At V

IN(MIN)

= 4.5V, the frequency will decrease and the P-channel will be conducting most of the time causing its power dissipation to increase. At V

IN(MIN)

= 4.5V:

Figure 6a. Continuous Mode Operation CT Waveform

Figure 6b. Burst Mode Operation CT Waveform

If Pin 2 is observed falling to ground at high output currents, it indicates poor decoupling or improper grounding. Refer to the Board Layout Checklist.

LTC1147-3.3

LTC1147-5/LTC1147L

WUU

APPLICATIO S I FOR A TIO

Board Layout Checklist

When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC1147 series. These items are also illustrated graphically in the layout diagram of Figure 7. Check the following in your layout:

1. Are the signal and power grounds segregated? The

LTC1147 ground (Pin 7) must return separately to a) the power and b) signal grounds. (a) returns to the source anode of the Schottky diode and (–) plate of CIN, which should have lead lengths as short as possible. The signal ground (b) connects to the (–) plate of C

OUT

2. Does the LTC1147 SENSE– (Pin 4) connect to a point

close to R

–

and the (+) plate of C

SENSE

BOLD LINES INDICATE HIGH CURRENT PATHS

The power ground

OUT

P-CH LR

1µF

3. Are the SENSE– and SENSE+ leads routed together with minimum PC trace spacing? The 1000pF capacitor between Pins 4 and 5 should be as close as possible to the LTC1147.

4. Does the (+) plate of CIN connect to the source of the P-channel MOSFET as closely as possible? This capacitor provides the AC current to the P-channel MOSFET.

5. Is the input decoupling capacitor (0.1µF/1µF) con- nected closely between VIN (Pin 1) and ground (Pin 7)? This capacitor carries the MOSFET driver peak currents.

6. On fixed output versions, is the SHDN (Pin 6) actively pulled to ground during normal operation? The SHDN pin is high impedance and must not be allowed to float.

SENSE

OUT

–

390pF

3300pF

LTC1147-3.3

LTC1147-5 (LTC1147L)

1

VIN

2



3



SENSE

PDRIVE

–

SENSE

GND

SHDN

1000pF

8

7



6



)



100pF

SHUTDOWN

Figure 7. LTC1147 Layout Diagram (See Board Layout Checklist)

For additional High Efficiency application circuits see Application Note 54.

OUTPUT DIVIDER REQUIRED WITH ADJUSTABLE VERSION ONLY

LTC1147 • F07

LTC1147-3.3 LTC1147-5/LTC1147L

TYPICAL APPLICATIO S

1

VIN

2

3

CT 120pF

CC 3300pF



SENSE

SUMIDA CDR74B-100LC

*

IRC LRC-LR2010-01-R068-F

3.3V Low Dropout High Efficiency Regulator

0.1µF

LTC1147L-3.3



–

PDRIVE

SENSE

GND

SHDN

0.01µF

8

7

6

SHUTDOWN

Si9433DY

L* 10µH

SENSE

0.068Ω

D1

MBRS130LT3

**



3.5V TO 12V

CIN 47µF 16V



OUT

100µF 10V

AVX



OUT

3.3V/1.25A

LTC1147 • F08

CT 300pF

1

VIN

2

3

SENSE

SUMIDA CDR74-221

*

IRC LRC-LR2010-01-R068-F

CC 3300pF



Precision Constant Current Source

0.1µF



LTC1147L

–

SENSE

0.01µF

PDRIVE

GND

SHDN

8

7

6



)



100pF

Si3455DV

L* 220µH

SENSE

0.22Ω

D1

MBRS130LT3

R1

1.2k

R2

**

6.8k

10V TO 14V

LTC1147 • F09

OUT

100µF 16V AVX

OUT



CIN 33µF 25V

(A)

OUT

1.0

0.8

0.6

0.4

0.2

VIN = 12.6V

OUT

vs V

For Figure 9

OUT

(V)

OUT

LTC1147 F10

TYPICAL APPLICATIO S

1

VIN

2

3

CT 120pF

CC 3300pF



SENSE

LTC1147L



0.1µF

–

PDRIVE

GND

SHDN

SENSE

0.01µF

2.5V/2A Regulator

8

7

6



)



I00pF

LTC1147-3.3

LTC1147-5/LTC1147L



3.5V TO 12V

CIN

OUT

220µF 10V × 2 AVX

15µF 25V × 2



Si9433DY

L* 10µH

SENSE

0.05Ω

D1

MBRD330

R1

49.9k 1%

R2

49.9k

**

COILTRONICS CTX10-4

*

IRC LR2512-01-0R050-G

PACKAGE DESCRIPTIO

0.300 – 0.325

(7.620 – 8.255)

0.065

(1.651)

0.009 – 0.015

(0.229 – 0.381)

+0.035

0.325

–0.015 +0.889

8.255

()

–0.381

*THESE DIMENSIONS DO NOT INCLUDE MOLD FLASH OR PROTRUSIONS. MOLD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.010 INCH (0.254mm)

TYP

0.100 ± 0.010

(2.540 ± 0.254)

Dimensions in inches (millimeters) unless otherwise noted.

N8 Package

8-Lead PDIP (Narrow 0.300)

(LTC DWG # 05-08-1510)

0.045 – 0.065

(1.143 – 1.651)

0.130 ± 0.005

(3.302 ± 0.127)

0.125

(3.175)

MIN

0.018 ± 0.003

(0.457 ± 0.076)



0.020

(0.508)

MIN

0.255 ± 0.015* (6.477 ± 0.381)



2.5V/2A

LTC1147 • F11

0.400* (10.160)

MAX

876

N8 1197

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 representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.

LTC1147-3.3 LTC1147-5/LTC1147L

TYPICAL APPLICATION

CT 220pF

COILTRONICS CTX20-4 

*

KRL SP-1/2-A1-OR050

CC 3300pF



PACKAGE DESCRIPTIO

0.010 – 0.020

(0.254 – 0.508)

0.008 – 0.010

(0.203 – 0.254)

*

DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH  

SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE

DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD  FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE  

× 45°

0°– 8° TYP

0.016 – 0.050

0.406 – 1.270

3.3V/2A Output High Efficiency Regulator

4V TO 14V

Si4431DY

L* 20µH

SENSE

0.05Ω

D1

MBRS130LT3

C 220µF 10V

AVX

**

LTC1147 • F12

OUT

1

2

3

VIN



LTC1147-3.3



SENSE

0.1µF

–

PDRIVE

GND

SHDN

SENSE

0.01µF

8

7

6

SHUTDOWN

Dimensions in inches (millimeters) unless otherwise noted.

S8 Package

8-Lead Plastic Small Outline (Narrow 0.150)

(LTC DWG # 05-08-1610)

0.053 – 0.069

(1.346 – 1.752)

0.014 – 0.019

(0.355 – 0.483)

0.004 – 0.010

(0.101 – 0.254)

0.050

(1.270)

TYP

0.228 – 0.244

(5.791 – 6.197)



CIN 22µF 25V × 2



OUT

3.3V/2A

0.189 – 0.197* (4.801 – 5.004)



0.150 – 0.157** (3.810 – 3.988)

SO8 0996

RELATED PARTS

PART NUMBER DESCRIPTION COMMENTS

LTC1142 Dual High Efficiency Synchronous Step-Down Switching Regulator Dual Version of LTC1148 LTC1143 Dual High Efficiency Step-Down Switching Regulator Controller Dual Version of LTC1147 LTC1147 High Efficiency Step-Down Switching Regulator Controller Nonsynchronous, 8-Lead, VIN ≤ 16V LTC1148 High Efficiency Step-Down Switching Regulator Controller Synchronous, VIN ≤ 20V LTC1149 High Efficiency Step-Down Switching Regulator Synchronous, VIN ≤ 48V, for Standard Threshold FETs LTC1159 High Efficiency Step-Down Switching Regulator Synchronous, VIN ≤ 40V for Logic Level MOSFETS LTC1174 High Efficiency Step-Down and Inverting DC/DC Converter 0.5A Switch, VIN ≤ 18.5V, Comparator LTC1265 High Efficiency Step-Down DC/DC Converter 1.2A Switch, VIN ≤ 13V, Comparator LTC1267 Dual High Efficiency Synchronous Step-Down Switching Regulators Dual Version of LTC1159 LTC1435 High Efficiency Low Noise Synchronous Step-Down Switching Regulator 16-Pin Narrow SO/SSOP; Constant Frequency

1147fd LT/TP 0698 REV D 2K • PRINTED IN USA

 LINEAR TECHNOLOGY CORPORATION 1993

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900 ● FAX: (408) 434-0507

●

www.linear-tech.com

Datasheet LTC1147L, LTC1147-5, LTC1147-3.3 Datasheet (Linear Technology)

Specifications and Main Features

Frequently Asked Questions

User Manual