LTC3206EUF#PBF Datasheet LTC3206EUF#PBF | P10-P12

LTC3206

3206f

White LED Brightness Control

The White LED displays (MAIN, SUB and AUX) have 16

individual brightness settings. The settings are exponen-

tially spaced to compensate for the nearly logarithmic

characteristic of human vision perception. The base of the

power settings is √2 . The off setting (0 power) is a special

case needed for shutdown.

The LTC3206 uses a subranging technique to control the

LED brightness with a combination of both DC level

control and pulse width modulation. Table 1 summarizes

the level control operation. The DC level of the LEDs will be

one of either three sub-range settings, 100%, 50% or 25%

of full scale. For example, if the full scale LED current is

programmed (via the IMS pin) to be 20mA, then the “on”

level of the LED will be either 20mA, 10mA or 5mA

respectively. The power to the LED will be the product of

the subrange (DC current) and the PWM setting. For

example, if an LED power of 2.25% is desired, then the

LTC3206 sets the sub range to 25% and the duty cycle to

8.8%. These settings are designed to optimize the effi-

ciency of the dual-mode LTC3206 power management

system while preserving LED color accuracy at low power

levels.

To achieve brightness control by purely DC means, only

the 100%, 50% or 25% power settings should be selected.

The DC current levels of the MAIN, SUB and AUX LEDs are

controlled by a precisely mirrored multiple of the current

at the I

pin. The I

pin servos to a fixed level of 0.6V so

the current is programmed simply by adding a resistor

from I

to ground.

The current that flows during the “on” time will follow the

relationship:

LED

= 400

••

where S is the subrange for the given power setting (it will

be either 25%, 50% or 100%, see Table 1) and R

is the

value of the resistor at the I

pin.

The average LED current (LED power level) will follow the

relationship:

AVG I

LED

() •

•=

−

400

where D is the decimal equivalent of the 4-bit digital code

programmed for the given display (0 to 15).

The PWM frequency is 1/1024 of the frequency of the

charge pump oscillator (typically 938Hz). During PWM,

the LED currents are soft-switched to minimize noise.

AUX LEDs

The AUX1 and AUX2 LEDs can be arbitrarily assigned to

either the MAIN or SUB display. Table 2 summarizes the

assignment possibilities. When an AUX pin is assigned to

a display, it will follow the power level (both DC and PWM)

set for that display.

Unused White LED Pins

The LTC3206 can power up to eight white LEDs (four for

the MAIN display, two for the SUB display and the two

flexible AUX pins), however, it is not necessary to use all

eight in each application.

Any of these LED pins can cause the LTC3206 to switch

from 1x mode to 1.5x charge pump mode if they drop out.

In fact, if an unused LED pin is left unconnected or

grounded, it

will

drop out and force the LTC3206 into

charge pump mode.

To avoid this problem, unused MAIN, SUB or AUX LED

pins can be disabled by connecting them to CPO. Power is

not wasted in this configuration. When the LED pin voltage

is within approximately 1V of CPO, its LED current is

switched off and only a small 10µA test current remains.

Figure 5 shows a block diagram of each of the MAIN, SUB

and AUX LED pins.

APPLICATIO S I FOR ATIO

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Figure 5. Internal MAIN, SUB and AUX LED Disable Circuit

–

CPO

MAIN1-MAIN4

SUB1, SUB2, AUX1, AUX2

10µA

3206 F05

ENABLE

LED

LTC3206

3206f

(see Figure 3 and Table 3) determines which display

ENRGB/S controls. When bit A2 is 0, the ENRGB/S pin

controls the RGB display. If it is set to 1, ENRGB/S controls

the SUB display.

To use the ENRGB/S pin, the I

C port must first be

configured to the desired setting. For example, if ENRGB/S

will be used to control the SUB display, then a non-zero

code must reside in the C3-C0 nibble of the I

C port and bit

A2 must be set to 1 (see Table 1). Now when ENRGB/S is

high (DV

), the SUB display will be on with the C3-C0

setting. When ENRGB/S is low, the SUB display will be off.

If no other displays are programmed to be on, the entire

chip will be in shutdown.

Likewise, if ENRGB/S will be used to enable the RGB

display, then a non-zero code must reside in one of the

RED, GREEN or BLUE nibbles of the serial port (A4-A7 or

B0-B7), and bit A2 must be 0. Now when ENRGB/S is high

(DV

), the RGB display will light with the programmed

color. When ENRGB/S is low, the RGB display will be off.

If no other displays are programmed to be on, the entire

chip will be in shutdown.

If bit A2 is set to 1 (SUB display control), then ENRGB/S

will have no effect on the RGB display. Likewise, if bit A2

is set to 0 (RGB display control), then ENRGB/S will have

no effect on the SUB display.

If the ENRGB/S pin is not used, it should be connected to

. It should not be grounded or left floating.

, CPO Capacitor Selection

The style and value of capacitors used with the LTC3206

determine several important parameters such as regulator

control-loop stability, output ripple and charge pump

strength. To reduce noise and ripple, it is recommended

that low equivalent series resistance (ESR) multilayer

ceramic capacitors be used on both V

and CPO. Tanta-

lum and aluminum capacitors are not recommended be-

cause of their high ESR. The value of the capacitor on CPO

directly controls the amount of output ripple for a given

load current. Increasing the size of this capacitor will

reduce the output ripple. The peak-to-peak output ripple is

approximately given by the expression:

RIPPLE

CPO

OSC CPO

P-P

≅

3•

The RED, GREEN and BLUE pins can also enable the

charge pump, however, since they each have individual

disable control they can be left floating or grounded if

unused.

RGB Illuminator Brightness Control

The RED, GREEN and BLUE LEDs can be individually set

to have a linear duty cycle ranging from 0/15 (off) to

15/15 (full on) with 1/15 increments in between. The

combination of 16 possible brightness levels gives the

RGB indicator LED a total of 4096 colors. Table 1 indicates

the decoding of the RED, GREEN and BLUE LEDs.

The full-scale currents in the RED, GREEN and BLUE LEDs

are controlled by the current at the I

RGB

pin in a similar

manner to those in the MAIN, SUB and AUX LEDs. The

RGB

pin also servos to 0.6V and the RGB LED currents are

a precise multiple of the I

RGB

current. The DC value of the

RGB display LED currents will follow the relationship:

RED GREENBLUE

RGB

= 400

where R

RGB

is the value of the resistor at the I

RGB

pin.

The average value of the current in the RED, GREEN and

BLUE LEDs will be:

AVG I

RED GREEN BLUE

RGB

()••

= 400

where D is the decimal equivalent of the 4-bit digital code

programmed for the given LED(0 to 15). Table 1 summa-

rizes the RED, GREEN and BLUE LED power settings.

The RED, GREEN and BLUE LEDs are pulse width modu-

lated at a frequency of 1/240 of the frequency of the charge

pump oscillator or about 4kHz.

ENRGB/S Pin

The ENRGB/S pin can be used to enable or disable the

LTC3206 without re-accessing the I

C port. This might be

useful to indicate an incoming phone call without waking

the microcontroller. ENRGB/S can be software pro-

grammed as an independent control for either the RGB

display or the SUB display. Control bit A2 in the serial port

APPLICATIO S I FOR ATIO

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LTC3206

3206f

where f

OSC

is the LTC3206’s oscillator frequency (typically

960kHz) and C

CPO

is the output charge storage capacitor

on CPO. Both the style and value of the output capacitor

can significantly affect the stability of the LTC3206. The

LTC3206 uses a linear control loop to adjust the strength

of the charge pump to match the current required at the

output. The error signal of this loop is stored directly on

the output charge storage capacitor. The charge storage

capacitor also serves to form the dominant pole for the

control loop. To prevent ringing or instability, it is impor-

tant for the output capacitor to maintain at least 0.6µF of

capacitance over all conditions. Likewise, excessive ESR

on the output capacitor will tend to degrade the loop

stability of the LTC3206. The closed-loop output resis-

tance of the LTC3206 is designed to be 0.4Ω. For a 100mA

load current change, the error signal will change by about

40mV. If the output capacitor has 0.4Ω or more of ESR,

the closed-loop frequency response will cease to roll off in

a simple one-pole fashion and poor load transient re-

sponse or instability could result. Multilayer ceramic chip

capacitors typically have exceptional ESR performance.

MLCC capacitors combined with a tight board layout, will

yield very good stability. As the value of C

CPO

controls the

amount of output ripple, the value of C

controls the

amount of ripple present at the input pin (V

). The input

current to the LTC3206 will be relatively constant while the

charge pump is on either the input charging phase or the

output charging phase but will drop to zero during the

clock nonoverlap 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 input current change times

the ESR. Therefore, ceramic capacitors are again recom-

mended for their exceptional ESR performance. Input

noise can be further reduced by powering the LTC3206

through a very small series inductor as shown in Figure 6.

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.

Flying Capacitor Selection

Figure 6. 10nH Inductor Used for Input Noise

Reduction (Approximately 1cm of Wire)

2.2µF0.1µF

GND

3206 F06

LTC3206

10nH

APPLICATIO S I FOR ATIO

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Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitors since

their voltage can reverse upon start-up of the LTC3206.

Ceramic capacitors should always be used for the flying

capacitors.

The flying capacitor controls the strength of the charge

pump. In order to achieve the rated output current it is

necessary to have at least 1µF 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 and

Y5V capacitors may also have a very poor voltage coeffi-

cient causing them to lose 60% or more of their capaci-

tance when the rated voltage is applied. Therefore, when

comparing different capacitors, it is often more appropri-

ate 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 capaci-

tor in a 0603 case may not provide any more capacitance

than a 0.22µF, 10V, X7R available in the same 0603 case.

The capacitor manufacturer’s data sheet should be con-

sulted to determine what value of capacitor is needed to

ensure minimum capacitances at all temperatures and

voltages.

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LTC3206EUF#PBF

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LED Lighting Drivers 400mA, I2C Multi-Display LED Driver

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