Datasheet

R
S
+
0.05
1.2 I
MAX
GATE10V/div
OUT10V/div
TIMER1V/div
I
IN
.5A/div
t-Time-200ms/div
TPS2490
TPS2491
SLVS503D –NOVEMBER 2003–REVISED JULY 2012 www.ti.com
Automatic Restart (Figure 17)
The TPS2491 automatically initiates a restart after a fault has caused it to turn off M1. Internal control circuits use
C
T
to count 16 cycles before re-enabling M1. This sequence repeats if the fault persists. The TIMER has a 1:10
charge-to-discharge current ratio, and uses a 1-V lower threshold. The fault-retry duty cycle specification
quantifies this behavior. This small duty cycle often reduces the average short-circuit power dissipation to levels
associated with normal operation and eliminates special thermal considerations for surviving a prolonged output
short.
Figure 17. TPS2491 Restart Cycle Timing
DESIGN PROCEDURE
This design procedure seeks to control the junction temperature of M1 under both static and transient conditions
by selecting the device’s package, cooling, R
DS(on)
, current limit, fault timeout, and power limit. The following
procedure assumes that a unit running at full load and maximum ambient temperature experiences a brief input
power interruption sufficient to discharge C
O
, but short enough to keep M1 from cooling. A full C
O
recharge then
takes place. Adjust this procedure to fit your application and design criteria. See SLVC033 for a calculation tool
to help with this process.
This procedure assumes that C
O
is the only load during inrush. Only simple first-order thermal models, natural
convection and a large PCB pad for M1 are assumed. The assumptions build generous safety margins into the
design to allow for the inherent inaccuracies of the models and variations of real-world conditions.
Other tools and applications information are available on the TI website that supplement the following procedure.
STEP 1. Choose R
S
Given the maximum operating current, I
MAX
, compute the current sense resistance, R
S
.
(4)
This equation allows for minimum current limit, a sense resistor tolerance of 5%, and 5% margin. Round the
result down to the nearest available standard value.
STEP 2. Choose M1
First select a V
DS
rating that allows for the maximum input voltage and transients. Next select an operating
R
DS(on)
, package, and cooling to control operating temperature. The following equation computes the value of
R
DS(on)(MAX)
at a junction temperature of T
J(MAX)
. Most manufacturers list R
DS(on)(MAX)
at 25°C and provide a
derating curve from which values at other temperatures can be derived. Compute the maximum allowable on-
resistance, R
DS(on)(MAX)
, using the equation:
14 Copyright © 2003–2012, Texas Instruments Incorporated