LMZ12003
SNVS636O –DECEMBER 2009–REVISED AUGUST 2015
www.ti.com
Typical Application (continued)
3. Program turnon time with soft-start capacitor selection
4. Select CO
5. Select CIN
6. Set operating frequency with RON
8.2.2.1 Enable Divider, RENT and RENB Selection
The enable input provides a precise 1.18-V band-gap rising threshold to allow direct logic drive or connection to
a voltage divider from a higher enable voltage such as Vin. The enable input also incorporates 90 mV (typical) of
hysteresis resulting in a falling threshold of 1.09 V. The maximum recommended voltage into the EN pin is 6.5 V.
For applications where the midpoint of the enable divider exceeds 6.5 V, a small Zener diode 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 turnon 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
where a lower boundary of operation must 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 LMZ12003 output rail. The two
resistors must be chosen based on the following ratio:
RENT / RENB = (VIN UVLO / 1.18V) – 1 (1)
The LMZ12003 demonstration and evaluation boards use 11.8 kΩfor RENB and 32.4 kΩfor RENT resulting in a
rising UVLO of 4.5 V. This divider presents 5.34 V to the EN input when the divider input is raised to 20 V.
The EN pin is internally pulled up to VIN and can be left floating for always-on operation.
8.2.2.2 Output Voltage Selection
Output voltage is determined by a divider of two resistors connected between VOand ground. The midpoint of
the divider is connected to the FB input. The voltage at FB is compared to a 0.8-V internal reference. In normal
operation an ON-time cycle is initiated when the voltage on the FB pin falls below 0.8 V. The main MOSFET ON-
time cycle causes the output voltage to rise and the voltage at the FB to exceed 0.8 V. As long as the voltage at
FB is above 0.8 V, ON-time cycles will not occur.
The regulated output voltage determined by the external divider resistors RFBT and RFBB is:
VO= 0.8 V × (1 + RFBT / RFBB) (2)
Rearranging terms; the ratio of the feedback resistors for a desired output voltage is:
RFBT / RFBB = (VO/ 0.8 V) - 1 (3)
These resistors must be chosen from values in the range of 1.0 kΩto 10.0 kΩ.
For VO= 0.8 V the FB pin can be connected to the output directly so long as an output preload resistor remains
that draws more than 20 µA. Converter operation requires this minimum load to create a small inductor ripple
current and maintain proper regulation when no load is present.
A feed-forward capacitor is placed in parallel with RFBT 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.
Table 1 lists the values for RFBT , RFBB , CFF and RON.
Table 1. Bill of Materials
REF DES DESCRIPTION CASE SIZE MANUFACTURER MANUFACTURER P/N
U1 SIMPLE SWITCHER PFM-7 Texas Instruments LMZ12003 TZ
Cin1 1-µF, 50-V, X7R 1206 Taiyo Yuden UMK316B7105KL-T
Cin2 10-µF, 50-V, X7R 1210 Taiyo Yuden UMK325BJ106MM-T
CO1 1-µF, 50-V, X7R 1206 Taiyo Yuden UMK316B7105KL-T
12 Submit Documentation Feedback Copyright © 2009–2015, Texas Instruments Incorporated
Product Folder Links: LMZ12003