Datasheet

Switch-Mode Controller
Current-Mode Control Loop
The advantages of current-mode control over volt-
age-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 cur-
rent-mode controller are reduced to that of a single-pole
system unlike the double pole in the voltage-mode control
scheme. The MAX16809 uses 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 com-
parator, 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
comparator, the output is high and the switch is turned off.
Undervoltage Lockout (UVLO)
The turn-on supply voltage for the MAX16809 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
MAX16809 operates with V
CC
down to 7.6V. Once V
CC
goes below 7.6V (typ), 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 typi-
cally. This allows for the MAX16809 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
V
CC
. The average current sourced to drive the external
MOSFET depends on the total gate charge (QG) and
operating frequency of the converter. The power dissipa-
tion in the MAX16809 is a function of the average output
drive current (I
DRIVE
). Use the following equation to cal-
culate 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 Amplier
The MAX16809 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 V
OUT
, FB, and AGND. Use the following for-
mula to set the output voltage:
OUT FB
R1
V 1 x V
R2

= +


where V
FB
= 2.5V.
Oscillator
The oscillator frequency is programmable using an exter-
nal 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 connected
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)]
where t
D
is in seconds, R
T
in ohms (Ω), and C
T
in
Farads (F).
Figure 1b. OUT_ _ Driver Internal Diagram
V+
W/L
OUT_ _
68W/L
945R
R
EST
1.23
R
1.23V
PGNDSET
MAX16809
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MAX16809 Integrated 16-Channel LED Driver with
Switch-Mode Boost and SEPIC Controller