MAX16807AUI datasheet(14/21 Pages) MAXIM

14 / 21 page

Integrated 8-Channel LED Drivers with

Switch-Mode Boost and SEPIC Controller

______________________________________________________________________________________

Switch-Mode Controller

Current-Mode Control Loop

The advantages of current-mode control over voltage-

mode control are twofold. First, there is the feed-for-

ward characteristic brought on by the controller’s ability

to adjust for variations in the input voltage on a cycle-

by-cycle basis. Second, the stability requirements of

the current-mode controller are reduced to that of a sin-

gle pole system unlike the double pole in the voltage-

mode control scheme. The MAX16807/MAX16808 use

a current-mode control loop where the output of the

error amplifier is compared to the current-sense voltage

(VCS). When the current-sense signal is lower than the

inverting input of the CPWM comparator, the output of

the comparator is low and the switch is turned on at

each clock pulse. When the current-sense signal is

higher than the inverting input of the CPWM compara-

tor, the output is high and the switch is turned off.

Undervoltage Lockout (UVLO)

The turn-on supply voltage for the MAX16807/

MAX16808 is 8.4V (typ). Once VCC reaches 8.4V, the

reference powers up. There is a 0.8V of hysteresis from

the turn-on voltage to the UVLO threshold. Once VCC

reaches 8.4V, the MAX16807/MAX16808 operate with

VCC down to 7.6V (typ). Once VCC goes below 7.6V,

the device is in UVLO. When in UVLO, the quiescent

supply current into VCC falls back to 32µA (typ), and

OUT and REF are pulled low.

MOSFET Driver

OUT drives an external n-channel MOSFET and swings

from AGND to VCC. Ensure that VCC remains below the

absolute maximum VGS rating of the external MOSFET.

OUT is a push-pull output with the on-resistance of the

pMOS typically 3.5Ω and the on-resistance of the

nMOS typically 4.5Ω. The driver can source 2A and

sink 1A typically. This allows for the MAX16807/

MAX16808 to quickly turn on and off high gate-charge

MOSFETs. Bypass VCC with one or more 0.1µF ceramic

capacitors to AGND, placed close to the VCC pin. The

average current sourced to drive the external MOSFET

depends on the total gate charge (QG) and operating

frequency of the converter. The power dissipation in the

MAX16807/MAX16808 is a function of the average out-

put drive current (IDRIVE). Use the following equation to

calculate the power dissipation in the device due to

IDRIVE:

IDRIVE = (QG x fSW)

PD = (IDRIVE + ICC) x VCC

where ICC is the operating supply current. See the

Typical Operating Characteristics for the operating

supply current at a given frequency.

Error Amplifier

The MAX16807/MAX16808 include an internal error

amplifier. The inverting input is at FB and the noninvert-

ing input is internally connected to a 2.5V reference.

Set the output voltage using a resistive divider between

output of the converter VOUT, FB, and AGND. Use the

following formula to set the output voltage:

where VFB = 2.5V.

Oscillator

The oscillator frequency is programmable using an

external capacitor and a resistor at RTCT (see RT and

CT in the Typical Operating Circuits). RT is connected

from RTCT to the 5V reference (REF), and CT is con-

nected from RTCT to AGND. REF charges CT through

RT until its voltage reaches 2.8V. CT then discharges

through an 8.3mA internal current sink until CT’s voltage

reaches 1.1V, at which time CT is allowed to charge

through RT again. The oscillator’s period is the sum of

the charge and discharge times of CT. Calculate the

charge time as follows:

tC = 0.57 x RT x CT

where tC is in seconds, RT in ohms (Ω), and CT in

Farads (F).

The discharge time is then:

tD = (RT x CT x 1000) / [(4.88 x RT) - (1.8 x 1000)]

OUT

⎛

⎝

⎜

⎞

⎠

⎟

V+

W/L

OUT_

68W/L

995R

REST

1.23

1 23V

PGND

SET

Figure 1c. OUT_ Driver Internal Diagram

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