Datasheet

LMZ14202H
www.ti.com SNVS691E –JANUARY 2011–REVISED OCTOBER 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 to an internal 0.8V reference. If the feedback voltage is below the reference, the high-side 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 high-side 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 LMZ14202H Application
The LMZ14202H is fully supported by Webench® which offers the following:
• Component selection
• Electrical simulation
• Thermal simulation
• Build-it prototype board for a reduction in design time
The following list of steps can be used to manually design the LMZ14202H 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 reference 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 the R
ENT
and R
ENB
divider shown in the Application Block Diagram 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 LMZ14202H output rail. The two resistors should be chosen based on the
following ratio:
R
ENT
/ R
ENB
= (V
IN-ENABLE
/ 1.18V) – 1 (1)
The EN pin is internally pulled up to VIN and can be left floating for always-on operation. However, it is good
practice to use the enable divider and turn on the regulator when V
IN
is close to reaching its nominal value. This
will guarantee smooth startup and will prevent overloading the input supply.
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 high-side 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 R
FBT
and R
FBB
is:
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