SNVS649E – JANUARY 2010 – REVISED MARCH 2013
COT CONTROL CIRCUIT OVERVIEW
Constant On Time control is based on a comparator and an on-time one shot, with the output voltage feedback
compared with an internal 0.8V reference. If the feedback voltage is below the reference, the main MOSFET is
turned on for a fixed on-time determined by a programming resistor R
ON
. R
ON
is connected to V
IN
such that on-
time is reduced with increasing input supply voltage. Following this on-time, the main MOSFET remains off for a
minimum of 260 ns. If the voltage on the feedback pin falls below the reference level again the on-time cycle is
repeated. Regulation is achieved in this manner.
Design Steps for the LMZ14201 Application
The LMZ14201 is fully supported by Webench® and offers the following: Component selection, electrical and
thermal simulations as well as the build-it board for a reduction in design time. The following list of steps can be
used to manually design the LMZ14201 application.
•
Select minimum operating V
IN
with enable divider resistors
•
Program V
O
with divider resistor selection
•
Program turn-on time with soft-start capacitor selection
•
Select C
O
•
Select C
IN
•
Set operating frequency with R
ON
•
Determine module dissipation
•
Layout PCB for required thermal performance
ENABLE DIVIDER, R
ENT
AND R
ENB
SELECTION
The enable input provides a precise 1.18V band-gap rising threshold to allow direct logic drive or connection to a
voltage divider from a higher enable voltage such as V
IN
. The enable input also incorporates 90 mV (typ) of
hysteresis resulting in a falling threshold of 1.09V. The maximum recommended voltage into the EN pin is 6.5V.
For applications where the midpoint of the enable divider exceeds 6.5V, a small zener can be added to limit this
voltage.
The function of this resistive divider is to allow the designer to choose an input voltage below which the circuit
will be disabled. This implements the feature of programmable under voltage lockout. This is often used in
battery powered systems to prevent deep discharge of the system battery. It is also useful in system designs for
sequencing of output rails or to prevent early turn-on of the supply as the main input voltage rail rises at power-
up. Applying the enable divider to the main input rail is often done in the case of higher input voltage systems
such as 24V AC/DC systems where a lower boundary of operation should be established. In the case of
sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than the
LMZ14201 output rail. The two resistors should be chosen based on the following ratio:
R
ENT
/ R
ENB
= (V
IN UVLO
/ 1.18V) – 1
(1)
The LMZ14201 demonstration and evaluation boards use 11.8k
Ω
for R
ENB
and 68.1k
Ω
for R
ENT
resulting in a
rising UVLO of 8V. This divider presents 6.25V to the EN input when the divider input is raised to 42V.
The EN pin is internally pulled up to VIN and can be left floating for always-on operation.
OUTPUT VOLTAGE SELECTION
Output voltage is determined by a divider of two resistors connected between V
O
and ground. The midpoint of
the divider is connected to the FB input. The voltage at FB is compared to a 0.8V internal reference. In normal
operation an on-time cycle is initiated when the voltage on the FB pin falls below 0.8V. The main MOSFET on-
time cycle causes the output voltage to rise and the voltage at the FB to exceed 0.8V. As long as the voltage at
FB is above 0.8V, on-time cycles will not occur.
The regulated output voltage determined by the external divider resistors RFBT and RFBB is:
V
O
= 0.8V * (1 + R
FBT
/ R
FBB
)
(2)
Rearranging terms; the ratio of the feedback resistors for a desired output voltage is:
R
FBT
/ R
FBB
= (V
O
/ 0.8V) - 1
(3)
These resistors should be chosen from values in the range of 1.0 kohm to 10.0 kohm.
12
Copyright © 2010–2013, Texas Instruments Incorporated
Product Folder Links:
