MAX16807 datasheet(13/19 Pages) MAXIM

13 / 19 page

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 uses 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 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 operates 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 3.5

Ω and the on-resistance of the

nMOS typically 4.5

Ω. The driver can source 2A and

sink 1A typically. This allows for the MAX16807 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 is a function of the average output 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 includes an internal error amplifier. The

inverting input is at FB and the noninverting 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 for-

mula 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)]

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

Farads (F).

OUT

⎛

⎝⎜

⎞

⎠⎟

V+

W/L

OUT_

68W/L

995R

REST

1.23

1.23V

PGND

SET

Figure 1c. OUT_ Driver Internal Diagram

Integrated 8-Channel LED Driver with

Switch-Mode Boost and SEPIC Controller

______________________________________________________________________________________

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