LINEAR TECHNOLOGY LTC3217 Technical data

FEATURES

■

Charge Pump Provides High Efficiency with
Automatic Mode Switching

■

Multimode Operation: 1x, 1.5x, 2x

■

Four Low Dropout LED Outputs

■

Up to 600mA Total Output Current

■

Independent Torch and Flash I

■

Low Noise Constant Frequency Operation*

■

PWM Brightness Control via the EN2 Pin

■

Low Shutdown Current: 4µA

■

Internal Soft-Start Limits Inrush Current During

and Enable Pins

SET

Start-Up and Mode Switching

■

Open/Short LED Protection

■

No Inductors

■

(3mm x 3mm) 16-Lead QFN Plastic Package

U

APPLICATIO S

■

Multi-LED Camera Light Supply for Cellphones/
DSCs/PDAs

, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
*Protected by U.S. Patents, including 6411531

LTC3217

600mA Low Noise

Multi-LED Camera Light

Charge Pump

U

DESCRIPTIO

The LTC®3217 is a low noise charge pump DC/DC con­verter designed to power four high current LEDs. The
LTC3217 requires only four small ceramic capacitors and
two current set resistors to form a complete LED power
supply and current controller.

Built-in soft-start circuitry prevents excessive inrush cur­rent during start-up and mode changes. High switching
frequency enables the use of small external capacitors.
Independent high and low current settings are programmed
by two external resistors. Shutdown mode and current
output levels are selected via two logic inputs.

The current through the LEDs is programmed via I
I

. In addition, the brightness can be controlled by

SET2

pulse width modulation of the EN2 pin.

The charge pump optimizes efficiency based on the volt­age across the LED current sources. The part powers up
in 1x mode and will automatically switch to boost mode
whenever any enabled LED current source begins to enter
dropout. The first dropout switches the part into 1.5x
mode and a subsequent dropout switches the part into 2x
mode. The LTC3217 resets to 1x mode whenever the part
is shut down.

SET1

and

TYPICAL APPLICATIO

C2

2.2µFC32.2µF

C1P C1M C2P C2M

V

BAT

C1

2.2µF

EN1 (TORCH)

EN2 (FLASH)

19.6k

V

EN1

EN2

1%

BAT

I

SET1ISET2

LTC3217

6.49k
1%

CPO

LED1

LED2

LED3

LED4
GND

C4

2.2µF

EN1

0
1
0
1

U

EN2 I

0

0 (SHUTDOWN)

0

25mA/LED

1

75mA/LED

1

100mA/LED

LED

The LTC3217 is available in a low profile 16-lead (3mm ×
3mm × 0.75mm) QFN package.

3217 TA01

AOT-2015HPW-1751B

100

100mA

90

80

70

/PIN) (%)

60

LED

50

40

30

EFFICIENCY (P

20

TOTAL OUTPUT CURRENT

10

/PIN

P

LED

LED = 2015 HPW AOT

0

3

3.2

Efficiency vs V

200mA

3.6

3.4
V

BAT

(V)

3.8

BAT

400mA

4

4.4

4.2

3217 G09

3217f

1

LTC3217

16 15 14 13

5 6 7 8

TOP VIEW

17

UD PACKAGE

16-LEAD (3mm × 3mm) PLASTIC QFN

9

10

11

12

4

3

2

1C1P

CPO

EN1

LED1

C2M

EN2

I

SET2

I

SET1

C2P

V

BAT

C1M

GND1

LED2

LED3

LED4

GND2

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

V

, CPO to GND ....................................... –0.3V to 6V

BAT

EN1, EN2 .................................... –0.3V to (V

(Note 2) ....................................................... 600mA

I

CPO

I

(Note 3) .................................................. 150mA

ILED1-4

CPO Short-Circuit Duration ............................. Indefinite

Operating Temperature Range (Note 4) ... –40°C to 85°C

Storage Temperature Range ..................–65°C to 125°C

BAT

+ 0.3V)

UUW

PACKAGE/ORDER I FOR ATIO

T

= 125°C, θJA = 68°C/W

EXPOSED PAD (PIN 17) IS GND MUST BE SOLDERED TO PCB

ORDER PART NUMBER

LTC3217EUD

Order Options Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF

Lead Free Part Marking: http://www.linear.com/leadfree/

Consult LTC Marketing for parts specified with wider operating temperature ranges.

JMAX

QFN PART MARKING

LBTQ

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

PARAMETER CONDITIONS MIN TYP MAX UNITS

V

Operating Voltage

BAT

I

Operating Current R

VBAT

V

Shutdown Current 4 µA

BAT

LED 1-4 Current

LED Current Ratio (I
LED Dropout Voltage Mode Switch Threshold, I

Mode Switching Delay EN1 Only 2.5 ms
LED Current Matching Any Two Outputs, I

Charge Pump (CPO)

1x Mode Output Voltage I

1.5x Mode Output Voltage I

2x Mode Output Voltage I
1x Mode Output Impedance 0.5 Ω

1.5x Mode Output Impedance V
2x Mode Output Impedance V
CLOCK Frequency

LED/ISET1/2

)I

The ● denotes the specifications which apply over the full operating

= 25°C. V

A

ISET1

I

CPO

I

CPO

I

CPO

LED

CPO

CPO

CPO

BAT

BAT

= 3.6V, C1 = C2 = C3 = C4 = 2.2µF, unless otherwise noted.

BAT

●

2.9 4.5 V

= R

= 20k, EN1 = EN2 = High

ISET2

= 0mA, 1x Mode 1 mA
= 0mA, 1.5x Mode 4 mA

= 0mA, 2x Mode 6 mA

= 25mA to 100mA

= 100mA 330 mV

LED

= 100mA 1 %

LED

= 0mA V
= 0mA 4.5 V

= 0mA 5.05 V

= 3.4V, V
= 3.2V, V

≤ 4.6V (Note 5) 2.8 Ω

CPO

≤ 5.1V (Note 5) 3.2 Ω

CPO

●

370 400 430 mA/mA

BAT

●

0.6 0.85 1.15 MHz

V

3217f

2

LTC3217

R

ISET

(kΩ)

0

I

LED

(mA)

40

80

120

20

60

100

10 20 30 40

3217 G03

505015253545

ELECTRICAL CHARACTERISTICS

temperature range, otherwise specifications are at T

The ● denotes the specifications which apply over the full operating

= 25°C. V

A

= 3.6V, C1 = C2 = C3 = C4 = 2.2µF, unless otherwise noted.

BAT

PARAMETER CONDITIONS MIN TYP MAX UNITS
EN1, EN2

Low Level Input Voltage (VIL)
High Level Input Voltage (VIH)
Input Current (IIH)

Input Current (IIL)
Minimum PWM On-Time EN2 Only

Maximum PWM Off-Time EN2 to Remain Enabled, EN1 = Low

I

, I

SET1

SET2

V

ISET1, ISET2

I

ISET1, ISET2

I

ISET1, ISET2

Current Range

Short-Circuit Current 800 µA

Note 1: Absolute Maximum Ratings are those values beyond which the life of
a device may become impaired.

Note 2: Based on charge pump long-term current density limitations.
Assumes an operating duty cycle of ≤ 10% under absolute maximum
conditions for durations less than 10 seconds. Maximum current for
continuous operation is 300mA.

Note 3: Based on LED current source long-term current density limitations.
Assumes an operating duty cycle of ≤ 10% under absolute maximum

I

LED1-4

= 12.5mA

conditions for durations less than 10 seconds. Maximum current for
continuous operation is 100mA.

Note 4: The LTC3217E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C ambient operating
temperature range are assured by design, characterization and correlation
with statistical process controls.

Note 5: 1.5x mode output impedance is defined as (1.5V
2x mode output impedance is defined as (2V

●

●

1.4 V

●

730µA

●

–1 1 µA

●

50 µs

●

●

1.175 1.215 1.255 V

●

31.25 375 µA

– V

BAT

CPO

BAT

)/I

0.4 V

1ms

– V

CPO

.

OUT

)/I

.

OUT

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LED Dropout Voltage
vs LED Current

400

V

= 3.6V

BAT

300

200

100

LED DROPOUT VOLTAGE (mV)

0

10

20 30 40 50

LED CURRENT (mA)

60 70 80 90 100

3217 G01

LED Pin Current
vs LED Pin Voltage

120

V

= 3.6V

BAT

100

80

60

40

LED PIN CURRENT (mA)

20

0

0

0.2 0.4 0.6 0.8
LED PIN VOLTAGE (V)

TA = 25°C unless otherwise noted.

vs R

I

LED

1.0

3217 G02

ISET

3217f

3

LTC3217

V

BAT

(V)

3

0

EFFICIENCY (P

LED

/PIN) (%)

10

30

40

50

100

70

3.4

3.8

4

3217 G09

20

80

90

60

3.2

3.6

4.2

4.4

TOTAL OUTPUT CURRENT
P

LED

/PIN

LED = 2015 HPW AOT

100mA

200mA

400mA

UW

TYPICAL PERFOR A CE CHARACTERISTICS

TA = 25°C unless otherwise noted.

1x Mode Switch Resistance
vs Temperature

0.65
I

= 200mA

CPO

0.60

SWITCH RESISTANCE (Ω)

0.55

0.50

0.45

0.40

0.35

SHUTDOWN CURRENT (µA)

BAT

V

7.5

6.5

5.5

4.5

3.5

2.5

1.5

–40

2.9

V

BAT

vs V

V

= 3.6V

BAT

V

= 3.3V

BAT

–15 10 35 60

TEMPERATURE (°C)

Shutdown Current

Voltage

BAT

TA = –40°C

TA = 85°C

3.3 3.7

3.1 3.5
V

VOLTAGE (V)

BAT

V

BAT

TA = 25°C

4.1

3.9

= 3.9V

4.3

3217 G04

3217 G07

85

4.5

1.5x Mode Charge Pump Open-Loop
Output Resistance vs Temperature
(1.5V

3.2

3.0

2.8

2.6

2.4

SWITCH RESISTANCE (Ω)

2.2

2.0
–40

– V

BAT

V

= 3V

BAT

= 4.2V

V

CPO

C2 = C3 = C4 = 2.2µF

–15 10 35 60

)/I

CPO

TEMPERATURE (°C)

CPO

Oscillator Frequency
vs Supply Voltage

980

970

960

950

940

930

920

FREQUENCY (kHz)

910

900

890

880

2.9

TA = 25°C

TA = –40°C

3.1 3.5

3.3

V

SUPPLY VOLTAGE (V)

BAT

3.7

TA = 85°C

3.9

4.1

4.3

3217 G05

3217 G08

85

4.5

2x Mode Charge Pump Open-Loop
Output Resistance vs Temperature
(2V

– V

BAT

3.8
V

BAT

V

CPO

3.6

C2 = C3 = C4 = 2.2µF

3.4

3.2

3.0

SWITCH RESISTANCE (Ω)

2.8

2.6

–40

Efficiency vs V

= 3V
= 4.8V

–15 10 35 60

)/I

CPO

CPO

TEMPERATURE (°C)

BAT

85

3217 G06

AC COUPLED

4

V

CPO

50mV/DIV

1.5x Mode CPO Output Ripple 2x Mode CPO Output Ripple

V

CPO

20mV/DIV

3217 G10

AC COUPLED

V
I
C

BAT

CPO

CPO

= 3.6V

= 400mA

= 2.2µF

500ns/DIV
I
C

BAT

CPO

CPO

= 3.6V

= 400mA

= 2.2µF

500ns/DIVV

3217 G11

V

CPO

1V/DIV

I

VBAT

500mA/DIV

EN1

5V/DIV

Charge Pump Mode Switching
and Input Current to Ground
(400mA Load)

0

V

1x

NO LOAD

= 3.6V

IN

1x

DROPOUT

1.5x

DROPOUT

1ms/DIV

2x

(5V)

3217 G12

3217f

UW

TYPICAL PERFOR A CE CHARACTERISTICS

LTC3217

TA = 25°C unless otherwise noted.

1.5x Mode CPO Voltage
vs Load Current

4.8
C2 = C3 = C4 = 2.2µF

4.6

3.4V

3.5V

3.6V

5000

3217 G13

U

4.4

4.2

4.0

CPO VOLTAGE (V)

3.8

3.6

3.3V

100 200 300 400

UU

3.2V

3.1V

V

= 3V

BAT

LOAD CURRENT (mA)

PI FU CTIO S

C1P, C2P, C1M, C2M (Pins 1, 16, 14, 12): Charge Pump
Flying Capacitor Pins. A 2.2µF X7R or X5R ceramic capaci-
tor should be connected from C1P to C1M and C2P to C2M.

CPO (Pin 2): Output of the charge pump used to power all
LEDs. This pin is enabled or disabled using the EN1 and
EN2 inputs. A 2.2µF X5R or X7R ceramic capacitor should
be connected to ground.

EN1, EN2 (Pins 3, 11): Inputs. The EN1 and EN2 pins are
used to select which current level is being supplied to the
LEDs, as well as to put the part into shutdown mode. The
truth table for these pins is as follows:

Truth Table

EN1 EN2 MODE

0 0 Shutdown

1 0 Low Current

0 1 High Current

1 1 Low + High Current

2x Mode CPO Voltage
vs Load Current

5.2
C2 = C3 = C4 = 2.2µF

5.1

5.0

4.9

4.8

4.7

4.6

CPO VOLTAGE (V)

4.5

4.4

4.3

4.2

100

0

3.3V

300

200

LOAD CURRENT (mA)

3.2V

V

BAT

3.1V

= 3V

400

3.6V

500

3217 G14

current to each LED output is set via the EN1 and EN2
inputs, and the programming resistors connected from
I

SET1

and I

to GND. Any of the four LED outputs can

SET2

be disabled by connecting the output directly to CPO.
10µA of current will flow through each directly connected
LED output. For single LED applications, all four LED pins
may be tied together and will accurately share current.

GND2 (Pin 8): Analog Ground. This pin should be
connected directly to a low impedance ground plane.

I

SET1/ISET2

Resistor Pins. The I

(Pins 9, 10): LED Current Programming

SET1

and I

pins will servo to 1.22V.

SET2

Resistors connected between each of these pins and GND
are used to set the high and low LED current levels.
Connecting a resistor 2k or less will cause the LTC3217 to
enter over-current shutdown.

GND1 (Pin 13): Charge Pump Ground. This pin should be
connected directly to a low impedance ground plane.

EN2 can be used for PWM of the LED currents. For proper
operation, the minimum pulse width should be 50µs and
the maximum low time should be 1ms if EN1 is low. If EN1
is high then the 1ms low time limitation does not apply.

LED1, LED2, LED3, LED4 (Pins 4, 5, 6, 7): LED1 to LED4
are the current source outputs. Each LED is connected in
between CPO (anodes) and LED1 – 4 (cathodes). The

(Pin 15): Supply Voltage. This pin should be

V

BAT

bypassed with a 2.2µF, or greater low ESR ceramic
capacitor.

Exposed Pad (Pin 17): This pad should be connected
directly to a low impedance ground plane for optimal
thermal and electrical performance.

3217f

5

LTC3217

BLOCK DIAGRA

W

1

C1P

850kHz

OSCILLATOR

V

BAT

15

14 16 12

C1M C2P C2M

CHARGE PUMP

GND1

CPO

13

2

–

+

ENABLE CP

+

CPO

+

I

SET1

9

MUX

I

SET2

10

–

1.22V

LED CURRENT

SOURCES

4

SHORT-CIRCUIT
PROTECTION

–

+
–

0.8V

LED1

LED2

LED3

LED4

4

5

6

7

EN1

3

250k

EN2

11

250k

PWM TIMING

CONTROL LOGIC

U

OPERATIO

Power Management

The LTC3217 uses a switched capacitor charge pump to
boost CPO to as much as 2 times the input voltage up to

5.1V. The part starts up in 1x mode. In this mode, V
connected directly to CPO. This mode provides maximum
efficiency and minimum noise. The LTC3217 will remain
in 1x mode until an LED current source drops out. Dropout
occurs when a current source voltage becomes too low
for the programmed current to be supplied. When dropout
is detected, the LTC3217 will switch into 1.5x mode. The
CPO voltage will then start to increase and will attempt to
reach 1.5x V

up to 4.5V. Any subsequent dropout will

BAT

BAT

is

GND2

8

THERMAL

SHUTDOWN

3217 BD

cause the part to enter the 2x mode. The CPO voltage will
attempt to reach 2x V

up to 5.05V. The LTC3217 will be

BAT

reset to 1x mode whenever the part is shut down.

A two phase non-overlapping clock activates the charge
pump switches. In the 2x mode the flying capacitors are
charged on alternate clock phases from V

to minimize

BAT

input current ripple and CPO voltage ripple. In 1.5x mode
the flying capacitors are charged in series during the first
clock phase and stacked in parallel on V

during the

BAT

second phase. This sequence of charging and discharging
the flying capacitors continues at a constant frequency
of 850kHz.

3217f

6

OPERATIO

I

VV

R

OUT

BAT CPO

OL

=

15.–

LTC3217

U

The LED currents are delivered by the four programmable
current sources. Three discrete current settings (Low, High,
Low + High) are available and may be selected via the EN1 and
EN2 pins. The values of these currents may be selected by
choosing the appropriate programming resistors. Each re­sistor is connected between the I

SET1

or I

pin to ground.

SET2

The resistor values required to attain the desired current
levels can be determined by Equation 1.

R

An R

SET

SETx

488

12

/

=

I

LEDx

(1)

resistor value of 2k or less (i.e., short-circuit) will
cause the LTC3217 to enter overcurrent shutdown mode.
This mode prevents damage to the part and external LEDs by
shutting down the high power sections of the part.

Each LED output can be disabled by connecting the pin
directly to CPO. Do not leave pins open as this will cause
dropout and subsequently mode changing.

Pulse Width Modulation Option

can be pulse width modulated to control the

EN2
LED brightness. The minimum allowable pulse width is 50µs
and the maximum low time is 1ms. Pulse width modulating
the EN2 input can be performed with EN1 high or low. If EN1
is high then there is no limitation on the EN2 low time. When
EN1 is low the part would normally go into shutdown
whenever EN2 goes low. Prevention of shutdown in this case
is achieved by an internal timer which delays shutdown until
EN2 has remained low for at least 1ms.

Charge Pump Strength and Regulation

Regulation is achieved by sensing the voltage at the CPO pin
and modulating the charge pump strength based on the error
signal. The CPO regulation voltages are set internally, and are
dependent on the charge pump modes as shown in Table 1.

When the LTC3217 operates in either 1.5x mode or 2x mode,
the charge pump can be modeled as a Thevenin-equivalent
circuit to determine the amount of current available from the
effective input voltage and effective open-loop output resis­tance, R

Table 1. Charge Pump Output Regulation Voltages

(Figure 1).

OL

CHARGE PUMP MODE REGULATED V

1.5x 4.5V

2x 5.05V

CPO

ROL is dependent on a number of factors including the
switching term, 1/(2f

OSC

• C

), internal switch resis-

FLY

tances and the non-overlap period of the switching circuit.
However, for a given R

, the amount of current available

OL

will be directly proportional to the advantage voltage of

1.5V

BAT

– V

for 1.5x mode and 2V

CPO

BAT

– V

CPO

for 2x
mode. Consider the example of driving white LEDs from
a 3.1V supply. If the LED forward voltage is 3.8V and the
current sources require 100mV, the advantage voltage for

1.5x mode is 3.1V • 1.5 - 3.8V – 0.1V or 750mV. Notice
that if the input voltage is raised to 3.2V, the advantage
voltage jumps to 900mV—a 20% improvement in avail­able strength.

From Figure 1, for 1.5x mode the available current is given by:

Soft-Start

Initially, when the part is in shutdown, a weak switch con­nects V

to CPO. This allows V

BAT

to slowly charge the CPO

BAT

output capacitor and prevent large charging currents to
occur.

The LTC3217 also employs a soft-start feature on its charge
pump to prevent excessive inrush current and supply droop
when switching into the step-up modes. The current available
to the CPO pin is increased linearly over a typical period of
125µs. Soft-start occurs at the start of both 1.5x and 2x
mode changes.

(2)

For 2x mode, the available current is given by:

VV

2–

I

OUT

=

BAT CPO

R

OL

(3)

Notice that the advantage voltage in this case is

3.1V • 2 – 3.8V – 0.1V = 2.3V. R

is higher in 2x mode

OL

but a significant overall increase in available current
is achieved.

3217f

7

LTC3217

OPERATIO

U

R

OL

+
–

BAT

OR 2V

BAT

+

CPO1.5V

–

3217 F01

Figure 1. Equivalent Open-Loop Circuit

V

in calculating Equations 2 and 3 is the minimum

CPO

required voltage for the LED and not the regulated voltage.

Typical values of R

as a function of temperature are shown

OL

in Figures 2 and 3.

3.2
V

= 3V

BAT

= 4.2V

V

CPO

3.0

C2 = C3 = C4 = 2.2µF

2.8

2.6

2.4

SWITCH RESISTANCE (Ω)

2.2

Shutdown Current

In shutdown mode all the circuitry is turned off and the
LTC3217 draws a very low current from the V
Furthermore, CPO is weakly connected to V

BAT

BAT

supply.

. The
LTC3217 enters shutdown mode when both the EN1 and
EN2 pins are brought low. EN1 and EN2 have 250k pull­down resistors to ground.

Thermal Protection

The LTC3217 has built-in overtemperature protection. At
internal die temperatures of around 150°C thermal shut­down will occur. This will disable all of the current sources
and charge pump until the die has cooled by about 15°C.
This thermal cycling will continue until the fault has
been corrected.

CPO Short-Circuit Protection

The LTC3217 has internal CPO short-circuit protection. An
internal comparator senses when CPO is below 0.8V
which forces the part into shutdown. A pull-up device
ensures start-up.

2.0
–40

–15 10 35 60

TEMPERATURE (°C)

85

3217 G05

Figure 2. 1.5x Mode Charge Pump Open-Loop Output
Resistance vs Temperature (1.5V

3.8

V

= 3V

BAT

= 4.8V

V

CPO

3.6

C2 = C3 = C4 = 2.2µF

3.4

3.2

3.0

SWITCH RESISTANCE (Ω)

2.8

2.6
–40

–15 10 35 60

TEMPERATURE (°C)

BAT

– V

CPO

3217 G06

)/I

CPO

85

Figure 3. 2x Mode Charge Pump Open-Loop Output
Resistance vs Temperature (2V

BAT

– V

CPO

)/I

CPO

Mode Switching

The LTC3217 will automatically switch from 1x mode to

1.5x mode and subsequently to 2x mode whenever a
dropout condition is detected at an LED pin. Dropout
occurs when a current source voltage becomes too low for
the programmed current to be supplied. The time from
dropout detection and mode switching is about 2.5ms.
This delay allows for the LED to warm up and reduce its
forward voltage which may remove the dropout condition.

If PWM is used on the EN2 pin, then the dropout time is
dependent on one to two PWM clock pulses.

The part is reset back to 1x mode when the part is shut
down (EN1 = EN2 = Low). The part may be set to the
desired output current level via EN1 and EN2. An internal
comparator will not allow the main switches to connect

and CPO in 1x mode until the voltage at the CPO pin

V

BAT

has decayed to less than or equal to the voltage at the
V

pin.

BAT

8

3217f

WUUU

APPLICATIO S I FOR ATIO

V

, CPO Capacitor Selection

BAT

The style and value of the capacitors used with the
LTC3217 determine several important parameters such
as regulator control loop stability, output ripple, charge
pump strength and minimum start-up time.

To reduce noise and ripple, it is recommended that low
equivalent series resistance (ESR) ceramic capacitors are
used for both CV

BAT

and C

capacitors are not recommended due to high ESR.

The value of C

directly controls the amount of output

CPO

ripple for a given load current. Increasing the size of C
will reduce output ripple at the expense of higher start-up
current. The peak-to-peak output ripple of the 1.5x mode
is approximately given by the expression:

I

I

RIPPLEP

Where f

=

-P

is the LTC3217 oscillator frequency or typi-

OSC

cally 850kHz and C

OUT

fC

(•)3

OSC CPO

is the output storage capacitor.

CPO

The output ripple in 2x mode is very small due to the fact
that load current is supplied on both cycles of the clock.

Both style and value of the output capacitor can significantly
affect the stability of the LTC3217. As shown in the Block
Diagram, the LTC3217 uses a control loop to adjust the
strength of the charge pump to match the required output
current. The error signal of the loop is stored directly on
the output capacitor. The output capacitor also serves as
the dominant pole for the control loop. To prevent ringing
or instability, it is important for the output capacitor to
maintain at least 1µF of capacitance over all conditions.

In addition, excessive output capacitor ESR will tend to
degrade the loop stability. The ESR of the output capacitor
should be <100mΩ. Multilayer ceramic chip capacitors
typically have exceptional ESR performance. MLCCs com­bined with a tight board layout will result in very good
stability. As the value of C
output ripple, the value of CV
ripple present at the input pin (V

. Tantalum and aluminum

CPO

controls the amount of

CPO

controls the amount of

BAT

). The LTC3217 input

BAT

CPO

(4)

LTC3217

current will be relatively constant while the charge pump
is either in the input charging phase or the output charging
phase but will drop to zero during the clock non-overlap
times. Since the non-overlap time is small (~25ns), these
missing “notches” will result in only a small perturbation
on the input power supply line. Note that a higher ESR
capacitor such as tantalum will have higher input noise
due to the higher ESR. Therefore, ceramic capacitors are
recommended for low ESR. Input noise can be further
reduced by powering the LTC3217 through a very small
series inductor as shown in Figure 4. A 10nH inductor will
reject the fast current notches, thereby presenting a nearly
constant current load to the input power supply. For
economy, the 10nH inductor can be fabricated on the PC
board with about 1cm (0.4") of PC board trace.

V

BAT

LTC3217

GND

3217 F04

Figure 4. 10nH Inductor Used for Input Noise Reduction
(Approximately 1cm of Board Trace)

Flying Capacitor Selection

Warning: Polarized capacitors such as tantalum or
aluminum should never be used for the flying capacitors
since their voltage can reverse upon start-up of the
LTC3217. Ceramic capacitors should always be used for
the flying capacitors.

The flying capacitors control the strength of the charge
pump. In order to achieve the rated output current it is
necessary to have at least 1.6µF of capacitance for each of
the flying capacitors. Capacitors of different materials
lose their capacitance with higher temperature and volt­age at different rates. For example, a ceramic capacitor
made of X7R material will retain most of its capacitance
from –40°C to 85°C whereas a Z5U or Y5V style capacitor
will lose considerable capacitance over that range. Z5U

3217f

9

LTC3217

WUUU

APPLICATIO S I FOR ATIO

and Y5V capacitors may also have a very poor voltage
coefficient causing them to lose 60% or more of their
capacitance when the rated voltage is applied. Therefore,
when comparing different capacitors, it is often more
appropriate to compare the amount of achievable capaci­tance for a given case size rather than comparing the
specified capacitance value. For example, over rated
voltage and temperature conditions, a 1µF, 10V, Y5V
ceramic capacitor in a 0603 case may not provide any
more capacitance than a 0.22µF, 10V, X7R available in the
same case. The capacitor manufacturer’s data sheet should
be consulted to determine what value of capacitor is
needed to ensure minimum capacitances at all
temperatures and voltages.

Table 2 shows a list of ceramic capacitor manufacturers
and how to contact them:

Table 2. Recommended Capacitor Vendors

AVX www.avxcorp.com

Kemet www.kemet.com

Murata www.murata.com

Taiyo Yuden www.t-yuden.com

Vishay www.vishay.com

Layout Considerations and Noise

The following guidelines should be followed when design­ing a PCB layout for the LTC3217:

1. The Exposed Pad should be soldered to a large copper
plane that is connected to a solid, low impedance
ground plane using plated through-hole vias for proper
heat sinking and noise protection.

2. Input and output capacitors must be placed close to the
part.

3. The flying capacitors must be placed close to the part.
The traces from the pins to the capacitor pad should be
as wide as possible.

4. V

, CPO traces must be wide to minimize inductance

BAT

and handle high currents.

5. LED pads must be large and connected to other layers
of metal to ensure proper LED heat sinking.

Power Efficiency

To calculate the power efficiency (η) of a white LED driver
chip, the LED power should be compared to the input
power. The difference between these two numbers repre­sents lost power whether it is in the charge pump or the
current sources. Stated mathematically, the power
efficiency is given by:

Due to its high switching frequency and the transient
currents produced by the LTC3217, careful board layout
is necessary. A true ground plane and short connections
to all capacitors will improve performance and ensure
proper regulation under all conditions.

The flying capacitor pins C1P, C2P, C1M and C2M will
have very high edge rate waveforms. The large dv/dt on
these pins can couple energy capacitively to adjacent PCB
runs. Magnetic fields can also be generated if the flying
capacitors are not close to the LTC3217 (i.e., the loop area
is large). To decouple capacitive energy transfer, a
Faraday shield may be used. This is a grounded PCB trace
between the sensitive node and the LTC3217 pins. For a
high quality AC ground, it should be returned to a solid
ground plane that extends all the way to the LTC3217.

P

LED

η =

P

IN

(5)

The efficiency of the LTC3217 depends upon the mode in
which it is operating. Recall that the LTC3217 operates as
a pass switch, connecting V

to CPO, until dropout is

BAT

detected at the LED pin. This feature provides the opti­mum efficiency available for a given input voltage and LED
forward voltage. When it is operating as a switch, the
efficiency is approximated by:

P

LED

η == =

P

(•)

VI

LED LED

(•)

IN

VIVV

BAT BAT

LED

BAT

(6)

since the input current will be very close to the sum of the
LED currents.

3217f

10

WUUU

APPLICATIO S I FOR ATIO

LTC3217

At moderate to high output power, the quiescent current
of the LTC3217 is negligible and the expression shown in
Equation 6 is valid.

Once dropout is detected at the LED pin, the LTC3217
enables the charge pump in 1.5x mode.

In 1.5x boost mode, the efficiency is similar to that of a
linear regulator with an effective input voltage of 1.5 times
the actual input voltage. This is because the input current
for a 1.5x charge pump is approximately 1.5 times the load
current. In an ideal 1.5x charge pump, the power
efficiency would be given by:

η

IDEAL

P

LED

== =

P

IN

VI

(•)

LED LED

VIVV

(•(.)•)(.•)15 15

BAT LED

LED

(7)

BAT

Similarly, in 2x boost mode, the efficiency is similar to that
of a linear regulator with an effective input voltage of 2

U

PACKAGE DESCRIPTIO

times the actual input voltage. In an ideal 2x charge pump,
the power efficiency would be given by:

η

IDEAL

P

LED

== =

P

IN

VI

(•)

LED LED

VIVV

(•()•)(•)22

BAT LED

LED

BAT

(8)

Thermal Management

For higher input voltages and maximum output current,
there can be substantial power dissipation in the LTC3217.
If the junction temperature increases above approxi­mately 150°C the thermal shutdown circuitry will auto­matically deactivate the output current sources and charge
pump. To reduce maximum junction temperature, a good
thermal connection to the PC board is recommended.
Connecting the Exposed Pad to a ground plane and
maintaining a solid ground plane under the device will
reduce the thermal resistance of the package and PC
board considerably.

3.50 ± 0.05

2.10 ± 0.05

0.70 ±0.05

1.45 ± 0.05
(4 SIDES)

0.25 ±0.05

0.50 BSC

RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS

NOTE:

1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2)

2. DRAWING NOT TO SCALE

3. ALL DIMENSIONS ARE IN MILLIMETERS

4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE
MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE

UD Package

16-Lead Plastic QFN (3mm × 3mm)

(Reference LTC DWG # 05-08-1691)

3.00 ± 0.10
(4 SIDES)

PIN 1
TOP MARK
(NOTE 6)

PACKAGE
OUTLINE

BOTTOM VIEW—EXPOSED PAD

0.75 ± 0.05

1.45 ± 0.10
(4-SIDES)

0.200 REF

0.00 – 0.05

5. EXPOSED PAD SHALL BE SOLDER PLATED

6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION
ON THE TOP AND BOTTOM OF PACKAGE

R = 0.115

TYP

15 16

PIN 1 NOTCH R = 0.20 TYP
OR 0.25 × 45° CHAMFER

0.40 ± 0.10

1

2

(UD16) QFN 0904

0.25 ± 0.05

0.50 BSC

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.

3217f

11

LTC3217

U

TYPICAL APPLICATIO

500mA Camera Flash with PWM Brightness Control

C2

2.2µFC32.2µF

C1P C1M C2P C2M

V

BAT

C1

2.2µF

1kHz

PWM

(5% TO 100% DC)

V

BAT

EN1

EN2

I

SET1ISET2

NC

LTC3217

3.92k
1%

CPO

LED1

LED2

LED3

LED4
GND

C4

2.2µF

3217 TA02

I

LED

500mA (MAX)

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ThinSOT is a trademark of Linear Technology Corporation.

Linear Technology Corporation

12

1630 McCarthy Blvd., Milpitas, CA 95035-7417

[phone] ● FAX: [phone] ● www.linear.com

© LINEAR TECHNOLOGY CORPORATION 2005

= 34V, IQ = 1.8mA,

OUT(MAX)

= 1.5V/1.8V, IQ = 180µA,

= 34V, IQ = 1.2mA,

= 34V, IQ = 1.9mA,

= 5V, IQ = 8mA,

= 5V, IQ = 6.5mA,

= 5V, IQ = 5mA,

= 0.8V, IQ = 20µA,

= 0.6V, IQ = 20µA,

LT/TP 0805 500 • PRINTED IN USA

3217f