LMZ12003TZ-ADJ/NOPB National Semiconductor, LMZ12003TZ-ADJ/NOPB Datasheet - Page 8

• Manufacturer Part Number: LMZ12003TZ-ADJ/NOPB
• Description: IC BUCK SYNC ADJ 3A TO-PMOD-7
• Manufacturer: National Semiconductor
• Series: SIMPLE SWITCHER®r
• Type: Point of Load (POL) Non-Isolated with UVLOr

Specifications of LMZ12003TZ-ADJ/NOPB

• Output: 0.8 ~ 6 V
• Number Of Outputs: 1
• Power (watts): 18W
• Mounting Type: Surface Mount
• Voltage - Input: 4.5 ~ 20V
• Package / Case: TO-PMOD-7, Power Module
• 1st Output: 0.8 ~ 6 VDC @ 3A
• Size / Dimension: 0.40" L x 0.54" W x 0.18" H (10.16mm x 13.77mm x 4.57mm)
• Power (watts) - Rated: 18W
• Operating Temperature: -40°C ~ 125°C
• Efficiency: 92%
• Approvals: EN
• Operating Temperature (max): 125C
• Operating Temperature (min): -40C
• Pin Count: 7
• Mounting: Surface Mount
• Package Type: TO-PMOD EP
• Case Length: 10.16mm
• Case Height: 4.57mm
• Screening Level: Automotive
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• 3rd Output: -
• 2nd Output: -
• Lead Free Status / Rohs Status: Compliant
• Other names: LMZ12003TZ-ADJTR

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General Description
The LMZ12003 SIMPLE SWITCHER power module is an
easy-to-use step-down DC-DC solution capable of driving up
to 3A load with exceptional power conversion efficiency, line
and load regulation, and output accuracy. The LMZ12003 is
available in an innovative package that enhances thermal
performance and allows for hand or machine soldering.
The LMZ12003 can accept an input voltage rail between 4.5V
and 20V and deliver an adjustable and highly accurate output
voltage as low as 0.8V. The LMZ12003 only requires three
external resistors and four external capacitors to complete the
power solution. The LMZ12003 is a reliable and robust design
with the following protection features: thermal shutdown, in-
put under-voltage lockout, output over-voltage protection,
short-circuit protection, output current limit, and allows startup
into a pre-biased output. A single resistor adjusts the switch-
ing frequency up to 1 MHz.
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 be-
low the reference, the main MOSFET is turned on for a fixed
on-time determined by a programming resistor R
connected to V
input supply voltage. Following this on-time, the main MOS-
FET 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 LMZ12003
Application
The LMZ12003 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 de-
sign time. The following list of steps can be used to manually
design the LMZ12003 application.
• Select minimum operating V
• Program V
• Program turn-on time with soft-start capacitor selection
• Select C
• Select C
• Set operating frequency with R
• Determine module dissipation
• Layout PCB for required thermal performance
ENABLE DIVIDER, R
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 Vin. 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 dis-
abled. 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
O
IN
O
with divider resistor selection
IN
such that on-time is reduced with increasing
ENT
AND R
IN
with enable divider resistors
ON
ENB
SELECTION
ON
. R
ON
is
8
rail rises at power-up. Applying the enable divider to the main
input rail is often done in the case of higher input voltage sys-
tems where a lower boundary of operation should be estab-
lished. 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
should be chosen based on the following ratio:
R
The LMZ12003 demonstration and evaluation boards use
11.8kΩ for R
LO of 4.5V. This divider presents 5.34V to the EN input when
the divider input is raised to 20V.
The EN pin is internally pulled up to VIN and can be left float-
ing for always-on operation.
OUTPUT VOLTAGE SELECTION
Output voltage is determined by a divider of two resistors
connected between V
vider 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 di-
vider resistors R
V
Rearranging terms; the ratio of the feedback resistors for a
desired output voltage is:
R
These resistors should be chosen from values in the range of
1.0 kohm to 10.0 kohm.
For V
rectly so long as an output preload resistor remains that draws
more than 20uA. 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 R
prove load step transient response. Its value is usually deter-
mined experimentally by load stepping between DCM and
CCM conduction modes and adjusting for best transient re-
sponse and minimum output ripple.
A table of values for R
the applications schematic.
SOFT-START CAPACITOR SELECTION
Programmable soft-start permits the regulator to slowly ramp
to its steady state operating point after being enabled, thereby
reducing current inrush from the input supply and slowing the
output voltage rise-time to prevent overshoot.
Upon turn-on, after all UVLO conditions have been passed,
an internal 8uA current source begins charging the external
soft-start capacitor. The soft-start time duration to reach
steady state operation is given by the formula:
t
This equation can be rearranged as follows:
C
Use of a 0.022μF capacitor results in 2.2 msec soft-start du-
ration. This is recommended as a minimum value.
As the soft-start input exceeds 0.8V the output of the power
stage will be in regulation. The soft-start capacitor continues
SS
O
ENT
FBT
SS
= 0.8V * (1 + R
= V
= t
/ R
/ R
O
REF
SS
= 0.8V the FB pin can be connected to the output di-
FBB
ENB
* 8 μA / 0.8V (5)
* C
= (V
= (V
ENB
SS
O
FBT
IN UVLO
and 32.4kΩ for R
/ Iss = 0.8V * C
/ 0.8V) - 1 (3)
FBT
and R
O
/ R
FBT
/ 1.18V) – 1 (1)
and ground. The midpoint of the di-
FBB
, R
FBB
) (2)
FBB
is:
SS
, C
ENT
/ 8uA (4)
FF
resulting in a rising UV-
and R
ON
is included in
FBT
to im-