Texas Instruments Evaluation Board for LM25085 - 42V Wide Vin Constant On-Time PFET Buck Switching Controller LM25085MYE LM25085MYEVAL/NOPB Datenbogen
Produktcode
LM25085MYEVAL/NOPB
t
OFF(CL)
=
(V
FB
x 0.93) + 0.28V
4 x 10
-6
x
((V
IN
/31) + 0.15)
Load Current Derating
5
Load Current Derating
Although the maximum load current for this evaluation board is specified as 5A, the data sheet for the
Si7465 PFET specifies a maximum continuous current of 3.2A. Since the input current, which is the
average current though Q1, increases as the input voltage is decreased, the load current must be derated
at low input voltage, see
Si7465 PFET specifies a maximum continuous current of 3.2A. Since the input current, which is the
average current though Q1, increases as the input voltage is decreased, the load current must be derated
at low input voltage, see
.
Figure 2. Maximum Load Current Derating
6
Operating at Low Voltage
When the input voltage is less than 5 V, the PFET (Q1) is on continuously (100% duty cycle) and the
output voltage is equal to the input voltage, minus voltage drops across the sense resistor, Q1, and the
inductor. As the input voltage is increased above 5V switching commences at the PGATE pin and the SW
node as the LM25085 regulates the output at 5 V. Since the LM25085 does not have a required minimum
off-time, the circuit transitions smoothly from 100% duty cycle to a regulated output.
output voltage is equal to the input voltage, minus voltage drops across the sense resistor, Q1, and the
inductor. As the input voltage is increased above 5V switching commences at the PGATE pin and the SW
node as the LM25085 regulates the output at 5 V. Since the LM25085 does not have a required minimum
off-time, the circuit transitions smoothly from 100% duty cycle to a regulated output.
7
Current Limit
The LM25085 peak current limit detection operates by sensing the voltage across either the R
DS(ON)
of Q1,
or a sense resistor (R5), during the on-time and comparing it to the voltage across R3 at the ADJ pin. The
current limit threshold is reached when the sensed voltage exceeds the voltage across R3. When current
limit is reached Q1 is immediately switched off. The current limit function is much more accurate and
stable over temperature when a sense resistor is used. The R
current limit threshold is reached when the sensed voltage exceeds the voltage across R3. When current
limit is reached Q1 is immediately switched off. The current limit function is much more accurate and
stable over temperature when a sense resistor is used. The R
DS(ON)
of a MOSFET has a wide process
variation and a large temperature coefficient.
Current sensing is disabled for a blanking time of
≊
100 ns at the beginning of each on-time to prevent
false triggering of the current limit comparator due to leading edge current spikes. After Q1 is turned off
due to current limit detection, Q1 is held off for a longer-than-normal off-time. The extended off-time is a
function of the input voltage and the voltage at the FB pin, as shown in
due to current limit detection, Q1 is held off for a longer-than-normal off-time. The extended off-time is a
function of the input voltage and the voltage at the FB pin, as shown in
. The current limit off-time
can be calculated from
:
(2)
The longer-than-normal forced off-time allows the inductor current to decrease to a low level before the
next on-time. This cycle-by-cycle monitoring, followed by a long forced off-time, provides effective
protection from output load faults over a wide range of operating conditions.
next on-time. This cycle-by-cycle monitoring, followed by a long forced off-time, provides effective
protection from output load faults over a wide range of operating conditions.
3
SNVA375B – October 2008 – Revised April 2013
AN-1905 LM25085 Evaluation Board
Copyright © 2008–2013, Texas Instruments Incorporated