10A µModule Step-Down Regulator with Advanced Input
and Load Protection
Design Note 527
Yan Liang
Introduction
Bus voltage surges pose a danger not only to the DC/DC
converter, but also to the load. Traditional overvoltage
protection schemes involving a fuse are not necessarily
fast enough, nor dependable enough to protect loads
such as FPGAs, ASICs and microprocessors. A better
solution is to accurately and quickly detect an overvoltage condition and respond by quickly disconnecting the
input supply while discharging excess voltage at the
load with a low impedance path. This is possible with
the protection features in the LTM® 4641.
Power and Protection
The LTM4641 is a 4.5V to 38V input, 0.6V to 6V output,
10A step-down μModule ® regulator with advanced
input and load protection options, including:
(A) Input Protection
• Undervoltage lockout, overvoltage shutdown with
latchoff thresholds
• N-Channel overvoltage power-interrupt MOSFET
driver
• Surge stopper capable with few external components
(B) Load Protection
• Robust, resettable latchoff overvoltage protection
• N-Channel overvoltage crowbar power MOSFET
driver
In addition, trip detection thresholds for the following
faults are customizable: input undervoltage, overtemperature, input overvoltage and output overvoltage.
Select fault conditions can be set for latchoff or hysteretic restart response—or disabled.
Output Overvoltage and Load Protection
The common output overvoltage protection scheme
used in the power supply and semiconductor control
IC industry is to turn on the synchronous (bottom)
MOSFET. This provides some overvoltage protection
during severe load current step-down events, but is not
very effective at protecting loads from genuine fault
conditions such as a short-circuited high side power
switching MOSFET. The LTM4641 provides best-inclass output overvoltage protection when an output
crowbar MOSFET (MCB) and an input series MOSFET
(MSP) are used together, as shown in Figure 1.
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4
VIN
4V TO 38V
4.5V START-UP
+
1V Load Protected from MTOP
Short-Circuit at 38VIN
MSP*
10µF
50V
×2
100µF
50V
3
VING VINGP VINH MTOP
VOUT
VINL
750k
CROWBAR
MBOT
fSET
UVLO
INTVCC
DRVCC
RUN
TRACK/SS
10nF
1
SW
VOSNS+
LTM4641
VOSNS–
VOUT
1V
100µF 10A
2
MCB**
×3
4
VINL, VINH (25V/DIV)
5.49k
3
2
CROWBAR (5V/DIV)
5.49k
1.1VOUT PEAK
LOAD
VOUT
(200mV/DIV)
GND
OVPGM
IOVRETRY OVLO FCB LATCH SGND+
SHORT-CIRCUIT APPLIED
DN527 F01a
1
5.6M
+SGND CONNECTS TO GND INTERNAL TO µMODULE REGULATOR
* MSP: (OPTIONAL) SERIES-PASS OVERVOLTAGE POWER INTERRUPT MOSFET, NXP PSMN014-60LS
** MCB: (OPTIONAL) OUTPUT OVERVOLTAGE CROWBAR MOSFET, NXP PH2625L
4µs/DIV
TESTED AT WORST-CASE CONDITION: NO LOAD
Figure 1. LTM4641 with Input Disconnect and Fast Crowbar Output Overvoltage Protection
06/14/527
DN527 F01b
MSP is placed between the input power source (VIN)
and the LTM4641's power stage input pins (VINH), and is
used as a resettable electronic power-interrupt switch.
When a fault condition, such as an output overvoltage
(OOV) condition, is detected by the LTM4641’s internal
circuitry, the gate of MSP is discharged within 2.6μs
(maximum) and MSP turns off. The input source supply is thus disconnected from the LTM4641’s power
stage input (VINH), preventing the hazardous (input)
voltage from reaching the precious load. LTM4641
also uses an independent reference voltage to generate an OOV threshold, separate from the control IC’s
bandgap voltage.
Figure 1 shows the CROWBAR and VOUT waveforms
when the top MOSFET MTOP fails, causing a shortcircuit between the VIN and SW nodes. CROWBAR
goes high within 500ns and turns on MCB to short
the output to ground. VOUT never exceeds 110% of the
specified output voltage.
Input Overvoltage and Undervoltage Protections
The LTM4641 has input undervoltage and overvoltage
protections, whose trip thresholds can be set by the
user. Please refer to Figure 2.
The UVLO pin feeds directly into the inverting input
of a comparator whose trip threshold is 0.5V. When
the UVLO pin falls below 0.5V, switching action is
inhibited; when the UVLO pin exceeds 0.5V, switching
action can resume. The IOVRETRY and OVLO pins each
feed directly into noninverting inputs of comparators
whose trip thresholds are 0.5V. When the IOVRETRY
pin exceeds 0.5V, switching action is inhibited; when
IOVRETRY falls below 0.5V, switching action can
resume. When the OVLO pin exceeds 0.5V, switching
action is inhibited; when OVLO subsequently falls below
0.5V, switching action cannot occur until the latch has
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been reset. These three pins give added flexibility to
tailor the behavior of the LTM4641.
VIN
+
CIN(MLCC)
10µF
×2
CIN(BULK)
VINH
VINL
RTUV
RBUV
RMOV
UVLO < 0.5V = OFF
UVLO
RHYST
HYST PULLS UP WHEN
ON, HYST PULLS DOWN
WHEN OFF
RTOV
IOVRETRY > 0.5V = OFF
OVLO > 0.5V = LATCHOFF
LTM4641
HYST
IOVRETRY
OVLO
SGND
RBOV
GND
AN527 F02
SGND CONNECTS TO GND INTERNAL TO MODULE.
KEEP SGND ROUTES/PLANES SEPARATE FROM GND
ON MOTHERBOARD
Figure 2. Circuit to Set the Input UVLO, IOVRETRY
and OVLO Thresholds
Efficiency
Figure 3 below shows the efficiency curves for the
LTM4641 for a typical 12V input voltage for the circuit
in Figure 1. With all the protection circuits, LTM4641
can still achieve high efficiency.
95
90
85
EFFICIENCY (%)
MCB is an external optional crowbar device residing
on VOUT. If the output voltage exceeds an adjustable
threshold —default value is 11% above nominal—
the LTM4641 pulls its CROWBAR output logic high
immediately (500ns response time, maximum) and
latches off its output voltage: the power stage becomes
high impedance, with both internal top and bottom
MOSFETs latched off. The CROWBAR output turns on
MCB, discharging the output capacitors and preventing
any further positive excursion of the output voltage.
80
6.0VOUT
5.0VOUT
3.3VOUT
2.5VOUT
1.8VOUT
1.5VOUT
1.2VOUT
1.0VOUT
0.9VOUT
75
70
65
60
0
1
2
3 4 5 6 7 8
OUTPUT CURRENT (A)
9
10
DN527 F03
Figure 3. Efficiency Curves of LTM4641
Conclusion
The LTM4641 μModule regulator monitors input
voltage, output voltage and temperature conditions.
It can provide comprehensive electrical and thermal
protection from excessive voltage stress for loads such
as processors, ASICs and high end FPGAs.
For applications help,
call (408) 432-1900, Ext. 3979
dn527 LT/AP 0614 111K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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 LINEAR TECHNOLOGY CORPORATION 2014