LMZ14202EXT_15 Datasheet, PDF (12/32 Pages) Texas Instruments – 2A SIMPLE SWITCHER Power Module With 42-V Maximum Input Voltage for Demanding Environments and Rugged Applications

LMZ14202EXT
SNVS665H – JUNE 2010 – REVISED OCTOBER 2015
7 Detailed Description
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7.1 Overview
7.1.1 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 0.8-V reference. If the feedback voltage is below the reference, the main 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 main 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.
7.2 Functional Block Diagram
Vin
5
SS
Linear reg
Css
3
EN
R_ON
2 RON
C_FF
Timer
6 FB
R_FBT
R_FBB
Regulator IC
GND
4
VIN 1
Cvcc
C_IN
0.47 éF
VOUT ⁷
6.8 éH
V_O
Co
Internal
Passives
7.3 Feature Description
7.3.1 Output Overvoltage Comparator
The voltage at FB is compared to a 0.92-V internal reference. If FB rises above 0.92 V, the ON-time is
immediately terminated. This condition is known as overvoltage protection (OVP). It can occur if the input voltage
is increased very suddenly or if the output load is decreased very suddenly. When OVP is activated, the top
MOSFET ON-times will be inhibited until the condition clears. Additionally, the synchronous MOSFET will remain
on until inductor current falls to zero.
7.3.2 Current Limit
Current limit detection is carried out during the OFF-time by monitoring the current in the synchronous MOSFET.
Referring to the Functional Block Diagram, when the top MOSFET is turned off, the inductor current flows
through the load, the PGND pin and the internal synchronous MOSFET. If this current exceeds 2.6 A (typical) the
current limit comparator disables the start of the next ON-time period. The next switching cycle will occur only if
the FB input is less than 0.8 V and the inductor current has decreased below 2.6 A. Inductor current is monitored
during the period of time the synchronous MOSFET is conducting. So long as inductor current exceeds 2.6 A,
further ON-time intervals for the top MOSFET will not occur. Switching frequency is lower during current limit due
to the longer OFF-time. It must also be noted that current limit is dependent on both duty cycle and temperature
as illustrated in the graphs in the Typical Characteristics.
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