Datasheet

LM2727, LM2737
SNVS205D –AUGUST 2002–REVISED MARCH 2013 www.ti.com
UVP/OVP
The output undervoltage protection and overvoltage protection mechanisms engage at 70% and 118% of the
target output voltage, respectively. In either case, the LM2727 will turn off the high side switch and turn on the
low side switch, and discharge the soft start capacitor through a MOSFET switch. The chip remains in this state
until the shutdown pin has been pulled to a logic low and then released. The UVP function is masked only during
the first charging of the soft start capacitor, when voltage is first applied to the V
CC
pin. In contrast, the LM2737 is
designed to continue operating during UVP or OVP conditions, and to resume normal operation once the fault
condition is cleared. As with the LM2727, the powergood flag goes low during this time, giving a logic-level
warning signal.
SHUT DOWN
If the shutdown pin SD is pulled low, the LM2727/37 discharges the soft start capacitor through a MOSFET
switch. The high side switch is turned off and the low side switch is turned on. The LM2727/37 remains in this
state until SD is released.
DESIGN CONSIDERATIONS
The following is a design procedure for all the components needed to create the circuit shown in Figure 26 in the
Example Circuits section, a 5V in to 1.2V out converter, capable of delivering 10A with an efficiency of 85%. The
switching frequency is 300kHz. The same procedures can be followed to create the circuit shown in Figure 26,
Figure 27, and to create many other designs with varying input voltages, output voltages, and output currents.
INPUT CAPACITOR
The input capacitors in a Buck switching converter are subjected to high stress due to the input current
waveform, which is a square wave. Hence input caps are selected for their ripple current capability and their
ability to withstand the heat generated as that ripple current runs through their ESR. Input rms ripple current is
approximately:
(6)
The power dissipated by each input capacitor is:
(7)
Here, n is the number of capacitors, and indicates that power loss in each cap decreases rapidly as the number
of input caps increase. The worst-case ripple for a Buck converter occurs during full load, when the duty cycle D
= 50%.
In the 5V to 1.2V case, D = 1.2/5 = 0.24. With a 10A maximum load the ripple current is 4.3A. The Sanyo
10MV5600AX aluminum electrolytic capacitor has a ripple current rating of 2.35A, up to 105°C. Two such
capacitors make a conservative design that allows for unequal current sharing between individual caps. Each
capacitor has a maximum ESR of 18mΩ at 100 kHz. Power loss in each device is then 0.05W, and total loss is
0.1W. Other possibilities for input and output capacitors include MLCC, tantalum, OSCON, SP, and POSCAPS.
INPUT INDUCTOR
The input inductor serves two basic purposes. First, in high power applications, the input inductor helps insulate
the input power supply from switching noise. This is especially important if other switching converters draw
current from the same supply. Noise at high frequency, such as that developed by the LM2727 at 1MHz
operation, could pass through the input stage of a slower converter, contaminating and possibly interfering with
its operation.
An input inductor also helps shield the LM2727 from high frequency noise generated by other switching
converters. The second purpose of the input inductor is to limit the input current slew rate. During a change from
no-load to full-load, the input inductor sees the highest voltage change across it, equal to the full load current
times the input capacitor ESR. This value divided by the maximum allowable input current slew rate gives the
minimum input inductance:
12 Submit Documentation Feedback Copyright © 2002–2013, Texas Instruments Incorporated
Product Folder Links: LM2727 LM2737