LM3535 datasheet(22/26 Pages) NSC

22 / 26 page

OUT – Output resistance. This parameter models the internal

losses of the charge pump that result in voltage droop at the

pump output V

OUT. Since the magnitude of the voltage droop

is proportional to the total output current of the charge pump,

the loss parameter is modeled as a resistance. The output

resistance of the LM3535 is typically 2.4

Ω (V

IN = 3.6V, TA =

25°C). In equation form:

VOUT = (1.5 × VIN) – [(NA× ILEDA + NB × ILEDB + NC × ILEDC) ×

OUT]

(eq. 2)

HR – Headroom constant. This parameter models the mini-

mum voltage required to be present across the current sinks

for them to regulate properly. This minimum voltage is pro-

portional to the programmed LED current, so the constant has

units of mV/mA. The typical k

HR of the LM3535 is 4mV/mA. In

equation form:

VOUT – VLEDx) > kHRx × ILEDx

(eq. 3)

Typical Headroom Constant Values

HRA = kHRB = kHRC = 4 mV/mA

The "I

LED-MAX" equation (eq. 1) is obtained from combining the

OUT equation (eq. 2) with the kHRx equation (eq. 3) and solv-

ing for I

LEDx. Maximum LED current is highly dependent on

minimum input voltage and LED forward voltage. Output cur-

rent capability can be increased by raising the minimum input

voltage of the application, or by selecting an LED with a lower

forward voltage. Excessive power dissipation may also limit

output current capability of an application.

Total Output Current Capability

The maximum output current that can be drawn from the

LM3535 is 200mA.

DRIVER TYPE

MAXIMUM Dxx CURRENT

DxA

25mA per DxA Pin

DxB

25mA per DxB Pin

D1C

25mA

PARALLEL CONNECTED AND UNUSED OUTPUTS

Connecting the outputs in parallel does not affect internal op-

eration of the LM3535 and has no impact on the Electrical

Characteristics and limits previously presented. The available

diode output current, maximum diode voltage, and all other

specifications provided in the Electrical Characteristics table

apply to this parallel output configuration, just as they do to

the standard LED application circuit.

All Dx current sinks utilize LED forward voltage sensing cir-

cuitry to optimize the charge-pump gain for maximum effi-

ciency. Due to the nature of the sensing circuitry, it is not

recommended to leave any of the Dx pins open when the

current sinks are enabled (ENx bits are set to '1'). Leaving Dx

pins unconnected will force the charge-pump into 3/2× mode

over the entire V

IN range negating any efficiency gain that

could have been achieved by switching to 1× mode at higher

input voltages.

If the D1B or D1C drivers are not going to be used, make sure

that the ENB and ENC bits in the general purpose register are

set to '0' to ensure optimal efficiency.

POWER EFFICIENCY

Efficiency of LED drivers is commonly taken to be the ratio of

power consumed by the LEDs (P

LED) to the power drawn at

the input of the part (P

IN). With a 3/2× - 1× charge pump, the

input current is equal to the charge pump gain times the output

current (total LED current). The efficiency of the LM3535 can

be predicted as follow:

LEDTOTAL = (VLEDA × NA × ILEDA) +

LEDB × NB × ILEDB) + (VLEDC × ILEDC)

IN = VIN × IIN

IN = VIN × (GAIN × ILEDTOTAL + IQ)

E = (P

LEDTOTAL ÷ PIN)

The LED voltage is the main contributor to the charge-pump

gain selection process. Use of low forward-voltage LEDs

(3.0V- to 3.5V) will allow the LM3535 to stay in the gain of 1×

for a higher percentage of the lithium-ion battery voltage

range when compared to the use of higher forward voltage

LEDs (3.5V to 4.0V). See the LED Forward Voltage Monitor-

ing section of this datasheet for a more detailed description

of the gain selection and transition process.

For an advanced analysis, it is recommended that power con-

sumed by the circuit (V

IN x IIN) for a given load be evaluated

rather than power efficiency.

POWER DISSIPATION

The power dissipation (P

DISS) and junction temperature (TJ)

can be approximated with the equations below. P

IN is the

power generated by the 3/2× - 1× charge pump, P

LED is the

power consumed by the LEDs, T

A is the ambient temperature,

and

JA is the junction-to-ambient thermal resistance for the

micro SMD 20-bump package. V

IN is the input voltage to the

LM3535, V

LED is the nominal LED forward voltage, N is the

number of LEDs and I

LED is the programmed LED current.

DISS = PIN - PLEDA - PLEDB - PLEDC

DISS= (GAIN × VIN × IGroupA + GroupB + GroupC ) - (VLEDA × NA ×

LEDA) - (VLEDB × NB × ILEDB) - (VLEDC × ILEDC)

J = TA + (PDISS x θJA)

The junction temperature rating takes precedence over the

ambient temperature rating. The LM3535 may be operated

outside the ambient temperature rating, so long as the junc-

tion temperature of the device does not exceed the maximum

operating rating of 110°C. The maximum ambient tempera-

ture rating must be derated in applications where high power

dissipation and/or poor thermal resistance causes the junc-

tion temperature to exceed 110°C.

THERMAL PROTECTION

Internal thermal protection circuitry disables the LM3535

when the junction temperature exceeds 150°C (typ.). This

feature protects the device from being damaged by high die

temperatures that might otherwise result from excessive pow-

er dissipation. The device will recover and operate normally

when the junction temperature falls below 125°C (typ.). It is

important that the board layout provide good thermal conduc-

tion to keep the junction temperature within the specified

operating ratings.

CAPACITOR SELECTION

The LM3535 requires 4 external capacitors for proper opera-

tion (C

1 = C2 = CIN = COUT = 1µF). Surface-mount multi-layer

ceramic capacitors are recommended. These capacitors are

small, inexpensive and have very low equivalent series re-

sistance (ESR <20m

Ω typ.). Tantalum capacitors, OS-CON

capacitors, and aluminum electrolytic capacitors are not rec-

ommended for use with the LM3535 due to their high ESR,

as compared to ceramic capacitors.

For most applications, ceramic capacitors with X7R or X5R

temperature characteristic are preferred for use with the

LM3535. These capacitors have tight capacitance tolerance

(as good as ±10%) and hold their value over temperature

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