MAX16807_07 Datasheet, PDF (14/21 Pages) Maxim Integrated Products – Integrated 8-Channel LED Drivers with Switch-Mode Boost and SEPIC Controller

Integrated 8-Channel LED Drivers with
Switch-Mode Boost and SEPIC Controller
V+
1.23V
68W/L
1.23
R_EST
W/L
995R
OUT_
R
SET
PGND
Figure 1c. OUT_ Driver Internal Diagram
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
(V_CS). 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 V_CC reaches 8.4V, the
reference powers up. There is a 0.8V of hysteresis from
the turn-on voltage to the UVLO threshold. Once V_CC
reaches 8.4V, the MAX16807/MAX16808 operate with
V_CC down to 7.6V (typ). Once V_CC goes below 7.6V,
the device is in UVLO. When in UVLO, the quiescent
supply current into V_CC 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 V_CC. Ensure that V_CC remains below the
absolute maximum V_GS 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 V_CC with one or more 0.1µF ceramic
capacitors to AGND, placed close to the V_CC pin. The
average current sourced to drive the external MOSFET
depends on the total gate charge (Q_G) and operating
frequency of the converter. The power dissipation in the
MAX16807/MAX16808 is a function of the average out-
put drive current (I_DRIVE). Use the following equation to
calculate the power dissipation in the device due to
I_DRIVE:
I_DRIVE = (Q_G x f_SW)
PD = (I_DRIVE + I_CC) x V_CC
where I_CC 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 V_OUT, FB, and AGND. Use the
following formula to set the output voltage:
V_OUT
=
⎛⎝⎜_{1 +}
R1⎞
R2 ⎠⎟
x
V_FB
where V_FB = 2.5V.
Oscillator
The oscillator frequency is programmable using an
external capacitor and a resistor at RTCT (see R_T and
C_T in the Typical Operating Circuits). R_T is connected
from RTCT to the 5V reference (REF), and C_T is con-
nected from RTCT to AGND. REF charges C_T through
R_T until its voltage reaches 2.8V. C_T then discharges
through an 8.3mA internal current sink until C_T’s voltage
reaches 1.1V, at which time C_T is allowed to charge
through R_T again. The oscillator’s period is the sum of
the charge and discharge times of C_T. Calculate the
charge time as follows:
t_C = 0.57 x R_T x C_T
where t_C is in seconds, R_T in ohms (Ω), and C_T in
Farads (F).
The discharge time is then:
t_D = (R_T x C_T x 1000) / [(4.88 x R_T) - (1.8 x 1000)]
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