MAX16808 Maxim Integrated Products, MAX16808 Datasheet - Page 14

• Manufacturer Part Number: MAX16808
• Description: (MAX16807 / MAX16808) Integrated 8-Channel LED Drivers
• Manufacturer: Maxim Integrated Products

Integrated 8-Channel LED Drivers with
Switch-Mode Boost and SEPIC Controller
Figure 1c. OUT_ Driver Internal Diagram
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
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.
The turn-on supply voltage for the MAX16807/
MAX16808 is 8.4V (typ). Once V
reference powers up. There is a 0.8V of hysteresis from
the turn-on voltage to the UVLO threshold. Once V
reaches 8.4V, the MAX16807/MAX16808 operate with
V
the device is in UVLO. When in UVLO, the quiescent
supply current into V
OUT and REF are pulled low.
OUT drives an external n-channel MOSFET and swings
from AGND to V
absolute maximum V
14
CC
CS
______________________________________________________________________________________
). When the current-sense signal is lower than the
down to 7.6V (typ). Once V
1.23V
R
1.23
EST
CC
SET
Undervoltage Lockout (UVLO)
68W/L
Switch-Mode Controller
. Ensure that V
Current-Mode Control Loop
GS
CC
rating of the external MOSFET.
V+
falls back to 32µA (typ), and
995R
W/L
CC
CC
CC
MOSFET Driver
remains below the
reaches 8.4V, the
goes below 7.6V,
PGND
R
OUT_
CC
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
capacitors to AGND, placed close to the V
average current sourced to drive the external MOSFET
depends on the total gate charge (Q
frequency of the converter. The power dissipation in the
MAX16807/MAX16808 is a function of the average out-
put drive current (I
calculate the power dissipation in the device due to
I
where I
Typical Operating Characteristics for the operating
supply current at a given frequency.
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
following formula to set the output voltage:
where V
The oscillator frequency is programmable using an
external capacitor and a resistor at RTCT (see R
C
from RTCT to the 5V reference (REF), and C
nected from RTCT to AGND. REF charges C
R
through an 8.3mA internal current sink until C
reaches 1.1V, at which time C
through R
the charge and discharge times of C
charge time as follows:
where t
Farads (F).
The discharge time is then:
DRIVE
T
T
t
in the Typical Operating Circuits ). R
until its voltage reaches 2.8V. C
D
= (R
:
CC
FB
C
T
T
is in seconds, R
= 2.5V.
is the operating supply current. See the
again. The oscillator’s period is the sum of
x C
PD = (I
V
T
OUT
x 1000) / [(4.88 x R
I
t
DRIVE
DRIVE
C
CC
= 0.57 x R
=
DRIVE
with one or more 0.1µF ceramic
⎛
⎜
⎝
). Use the following equation to
= (Q
1
OUT
+
+ I
T
G
, FB, and AGND. Use the
R
R
in ohms (Ω), and C
CC
2
1
T
x f
T
⎞
⎟
⎠
x C
) x V
SW
is allowed to charge
x V
T
T
Error Amplifier
)
T
) - (1.8 x 1000)]
CC
FB
G
then discharges
T
) and operating
. Calculate the
T
is connected
Oscillator
CC
T
T
’s voltage
T
pin. The
through
is con-
T
and
T
in