MAX16809
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 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 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 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 (Q
G
) 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 Amplifier
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:
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)]
where t
D
is in seconds, R
T
in ohms (Ω), and C
T
in
Farads (F).
