MAX16812ATI/V+T Datasheet MAX16812ATI+, MAX16812ATI+T, MAX16812ATI/V+T | P10-P12

MAX16812

Integrated High-Voltage LED Driver

with Analog and PWM Dimming Control

10 ______________________________________________________________________________________

Undervoltage Lockout

The MAX16812 features an adjustable UVLO through

the enable input (EN). Connect EN directly to IN to use

the 5V default UVLO. Connect EN to IN through a resis-

tive divider to ground to set the UVLO threshold. The

MAX16812 is enabled when V

exceeds the 1.38V

(typ) threshold.

Calculate the EN UVLO resistor-divider values as fol-

lows (see Figure 2):

where R

UV1

is in the 20kΩ range, V

is the 1.38V (typ)

EN threshold voltage, and V

UVLO

is the desired input-

voltage UVLO threshold in volts. Due to the 100mV hys-

teresis of the UVLO threshold, capacitor C

required to prevent chattering at the UVLO threshold

due to line impedance drops at power-up and during

dimming. If the undervoltage setting is very close to the

required minimum operating voltage, there can be

jumps in the voltage at IN while dimming. C

should

be large enough to limit the ripple on EN to less than

100mV (EN hysteresis) under these conditions so that it

does not turn on and off due to the ripple on IN.

Soft-Start

The soft-start feature of the MAX16812 allows the LED

string current to ramp up in a controlled manner, thus

minimizing output-voltage overshoot. While the part is in

UVLO, C

REF

is discharged (Figure 3). Upon coming out

of UVLO, an internal current source starts charging C

REF

during the soft-start cycle. Use the following equation to

calculate total soft-start time:

where I

REF

is 40µA, C

REF

is in µF, and t

is in sec-

onds. Operation begins when REF ramps above 0.6V.

Once the soft-start is complete, REF is regulated to

1.238V, the internal voltage reference.

Low-Side Internal

Switching MOSFET Driver Supply (L_REG)

L_REG is the regulated (5.2V) internal supply voltage

capable of delivering 20mA. L_REG provides power to

the gate drive of the internal switching power MOSFET.

L_REG

is referenced to AGND. Connect a 3.3µF

ceramic capacitor from L_REG to AGND.

High-Side Regulator (H_REG)

H_REG is a low-dropout linear regulator referenced to

LV. H_REG provides the gate drive for the external

n-channel dimming MOSFET and also powers up the

MAX16812’s LED current-sense circuitry. Bypass

H_REG to LV with a 1µF ceramic capacitor.

ST REF

REF

=×

1 238

R R x

V- V

UV1 UV2

UVLO EN

⎛

⎝

⎜

⎞

⎠

⎟

MAX16812

AGND

UV2

UV1

Figure 2. UVLO Threshold Setting

MAX16812

AGND

REF

Figure 3. Soft-Start Setting

MAX16812

Integrated High-Voltage LED Driver

with Analog and PWM Dimming Control

______________________________________________________________________________________ 11

High-Side Current-Sense Output (CS_OUT)

A high-side transconductance amplifier converts the

voltage across the LED current-sense resistor (R

)

into an internal current output. This current flows

through an internal resistor connected to AGND. The

voltage gain for the LED current-sense signal is 4. The

amplified signal is then buffered and connected

through an internal switch to CS_OUT.

Internal Error Amplifier

The MAX16812 includes a built-in voltage-error amplifi-

er, which can be used to close the feedback loop. The

internal LED current-sense output signal is buffered

internally and then connected to CS_OUT through an

internal switch. CS_OUT is connected to the inverting

input (FB) pin of the error amplifier through a resistor.

See Figures 4 and 5. The reference voltage for the out-

put current is connected to REFI, the noninverting input

of the error amplifier. When the internal dimming signal

is low, COMP is disconnected from the output of the

error amplifier and CS_OUT is simultaneously discon-

nected from the buffered LED current-sense output sig-

nal (Figure 5). When the internal dimming signal is high,

the output of the op amp is connected to COMP and

CS_OUT is connected to the buffered LED current-

sense signal at the same time (Figure 4). This enables

the compensation capacitor to hold the charge when

the DIM signal has turned off the internal switching

MOSFET gate drive. To maintain the charge on the

compensation capacitors C

COMP1

and C

COMP2

, the

capacitors should be of the low-leakage ceramic type.

When the internal dimming signal is enabled, the voltage

on the compensation capacitor forces the converter into

steady state almost instantaneously. The voltage on

COMP is subtracted from the internal slope compensa-

tion signal and is then connected to one of the inputs of

the PWM comparator. The PWM comparator input is of

the CMOS type with very low bias currents.

REFI

COMP

OUT

COMP2

COMP1

COMP2

STATE A

Figure 4. Internal Error Amplifier Connection (Dimming Signal High)

REFI

COMP

OUT

STATE B

COMP2

COMP1

COMP2

Figure 5. Internal Error Amplifier Connections (Dimming Signal Low)

MAX16812

Integrated High-Voltage LED Driver

with Analog and PWM Dimming Control

12 ______________________________________________________________________________________

Analog Dimming

The MAX16812 offers analog dimming of the LED cur-

rent by allowing the application of an external voltage

at REFI. The output current is proportional to the volt-

age at REFI. Use a potentiometer from REF or directly

apply an external voltage source at REFI.

PWM Comparator

The PWM comparator uses the instantaneous switch

current, the error-amplifier output, and the slope com-

pensation to determine when the gate drive DRV to the

internal n-channel switching MOSFET turns off. In nor-

mal operation, gate drive DRV to the n-channel MOS-

FET turns off when:

x R

SRC

≥ V

COMP

- V

OFFSET

- V

SCOMP

where I

is the current through the internal n-channel

switching MOSFET, R

SRC

is the switch current-sense

resistor, V

COMP

is the output voltage of the internal

amplifier, V

OFFSET

is the internal DC offset, which is a

drop, and V

SCOMP

is the ramp function that starts

at zero and slews at the programmed slew rate (SR).

Internal Switching MOSFET Current Limit

The current-sense resistor (R

SRC

), connected between

the source of the internal MOSFET and ground, sets the

current limit. The SRC input has a voltage trip level

SRC

) of 600mV for the cycle-by-cycle current limit. Use

the following equation to calculate the value of R

SRC

where I

LXLIM

is the peak current that flows through the

switching MOSFET at full load and low line. When the

voltage produced by this current (through the current-

sense resistor) exceeds the current-limit (ILIM) com-

parator threshold, the MOSFET driver (DRV) quickly

terminates the current on-cycle. The 200ns leading-

edge blanking circuit suppresses the leading-edge

spike on the current-sense waveform from appearing at

the current-limit comparator. There is also a hiccup

comparator (HICCUP) that limits the peak current in the

internal switch set at twice the peak limit setting.

Internal n-Channel

Switching MOSFET Driver (DRV)

L_REG provides power for the DRV output. Connect a

resistor from DRV to gate GT of the internal switching

MOSFET to control the switching MOSFET rise and fall

times, if necessary.

External Dimming

MOSFET Gate Drive (DGT)

DGT is the gate drive to the external dimming MOSFET

referenced to LV. H_REG provides the power to the

gate drive.

Overvoltage Protection

The overvoltage protection (OVP) comparator com-

pares the voltage at OV with a 1.238V (typ) internal ref-

erence. When the voltage at OV exceeds the internal

reference, the OVP comparator terminates PWM switch-

ing and no further energy is transferred to the load.

Connect OV to HV through a resistive voltage-divider to

ground to set the overvoltage threshold at the output.

Setting the Overvoltage Threshold

Connect OV to HV or to the high-side of the LEDs

through a resistive voltage-divider to set the overvolt-

age threshold at the output (Figure 6).

SRC

LXLIM

MAX16812

LED+

AGND

OV1

OV2

MAX16812

LED+

AGND

OV1

OV2

Figure 6. OVP Setting

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21

MAX16812ATI/V+T

Mfr. #:

Buy MAX16812ATI/V+T

Manufacturer:

Maxim Integrated

Description:

LED Lighting Drivers w/Analog & PWM Dimming Control

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MAX16812ATI/V+T

MAX16812ATI/V+T

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