Datasheet

LMZ14203H
www.ti.com SNVS692D –JANUARY 2011–REVISED OCTOBER 2013
DESIGN STEPS FOR THE LMZ14203H APPLICATION
The LMZ14203H 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 LMZ14203H application.
1. Select minimum operating V
IN
with enable divider resistors
2. Program V
O
with divider resistor selection
3. Program turn-on time with soft-start capacitor selection
4. Select C
O
5. Select C
IN
6. Set operating frequency with R
ON
7. Determine module dissipation
8. 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 Figure 52 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 LMZ14203H 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 ensure 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:
V
O
= 0.8V x (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 kΩ to 50 kΩ.
A feed-forward capacitor is placed in parallel with R
FBT
to improve load step transient response. Its value is
usually determined experimentally by load stepping between DCM and CCM conduction modes and adjusting for
best transient response and minimum output ripple.
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