LMZ22005 datasheet(18/34 Pages) TI

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LMZ22005

SNVS686J – MARCH 2011 – REVISED AUGUST 2015

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Typical Application (continued)

8.2.2 Detailed Design Procedure

8.2.2.1 Design Steps

The LMZ22005 is fully supported by WEBENCH which offers: component selection, electrical and thermal

simulations. Additionally there is are evaluation and demonstration boards that may be used a starting point for

design. The following list of steps can be used to quickly design the LMZ22005 application.

1. Select minimum operating VIN with enable divider resistors

2. Program VO with resistor divider selection

3. Select CO

4. Select CIN

5. Determine module power dissipation

6. Layout PCB for required thermal performance

8.2.2.2 Enable Divider, RENT, RENB and RENH Selection

Internal to the module is a 2-M

Ω pullup resistor connected from VIN to Enable. For applications not requiring

precision undervoltage lockout (UVLO), the Enable input may be left open circuit and the internal resistor will

always enable the module. In such case, the internal UVLO occurs typically at 4.3 V (VIN rising).

In applications with separate supervisory circuits Enable can be directly interfaced to a logic source. In the case

of sequencing supplies, the divider is connected to a rail that becomes active earlier in the power-up cycle than

the LMZ22005 output rail.

Enable provides a precise 1.279-V threshold to allow direct logic drive or connection to a voltage divider from a

higher enable voltage such as VIN. Additionally there is 21 μA (typical) of switched offset current allowing

programmable hysteresis. See Figure 47.

The function of the enable divider is to allow the designer to choose an input voltage below which the circuit will

be disabled. This implements the feature of programmable UVLO. The two resistors must be chosen based on

the following ratio:

RENT / RENB = (VIN UVLO / 1.279 V) – 1

(1)

The LMZ22005 typical application shows 12.7 k

Ω for RENB and 42.2 kΩ for RENT resulting in a rising UVLO of

5.46 V.

NOTE

A midpoint 5.1-V Zener clamp is present to allow setting UVLO to cover an extended

range of operation. The Zener clamp is not required if the target application prohibits the

maximum Enable input voltage from being exceeded.

Additional enable voltage hysteresis can be added with the inclusion of RENH. It may be possible to select values

for RENT and RENB such that RENH is a value of zero allowing it to be omitted from the design.

Rising threshold can be calculated as follows:

VEN(rising) = 1.279 ( 1 + RENT|| 2 meg/ RENB)

(2)

Whereas falling threshold level can be calculated using:

VEN(falling) = VEN(rising) – 21 µA ( RENT|| 2 meg || RENTB + RENH )

(3)

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