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Short-Circuit Protection for Boost Regulators – Design Note 154
Jeff Witt
The basic boost regulator topology provides no shortcircuit protection. When the output is pulled low, a large
current can flow from the input through the inductor
and catch diode, limited only by the series resistance
of these parts. The result may be damage to the boost
regulator, the load or the power source. This design note
presents several solutions to this potential problem.
Short-Circuit Protection and Load Disconnect
with the LTC1477
The LTC ®1477 protected high side switch contains
an internal, low loss NMOS power switch. It provides
protection to external circuits in two ways: switch
current is limited to 0.85A, 1.5A or 2A (depending on
configuration) and thermal shutdown circuitry turns
the switch off when power dissipation raises the die
temperature to 130°C. In Figure 1, the LTC1477 protects
an LT®1304 micropower boost regulator and its load
from excessive currents. An increasing load current will
first be limited to 0.85A; as more power is dissipated
in the switch, the LTC1477 will cycle the switch to limit
average current and dissipation to an acceptable level.
The enable pin can be used to disconnect the output.
In this circuit the LT1304 uses its low-battery detector
to shut itself down and disconnect the load when the
battery voltage drops below 2.7V.
100μF
16V
562k
1%
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
220k
NC
VIN
SW
ILIM
SENSE
LBI
432k
1%
(5V)
+
LT1304CS8-5
SINGLE*
Li-Ion
CELL
Current-Limited Boost Regulator
It may be desirable to limit, rather than interrupt, output
current to a heavy load. Figure 2 shows a 2-cell to 5V
boost converter with output current limited to 150mA.
The LT1304 generates 5V at the source of Q1, which,
with its gate grounded through R1 and R2, turns on
to supply current to the output. As the load current
increases, the voltage across RSENSE reaches 0.6V,
Q2 turns on and current through R2 raises the voltage
at the feedback pin; the LT1304 begins to regulate the
output current to 150mA. When the output voltage
is pulled lower than the input voltage, the LT1304 is
no longer able to control the output current, and Q2
regulates the current by pulling up on Q1’s gate. Note
that Q1 must dissipate approximately 0.4W when the
output is shorted to ground. An extra transistor (Q3)
22μH** MBRS130LT3
(2.7V to 4.2V)
+
For higher power applications, the VIN2 and VIN3 pins
of the LTC1477 should also be connected to the input.
This raises the current limit to 2A; switch resistance
will be just 0.07Ω. The LTC1477 operates from 2.7V
to 5.5V and is packaged in an SO-8. The LTC1478 is a
dual version available in a 16-lead SO.
SHDN
+
100μF†
10V
5V
1μF
VOUT
VOUT
VIN1
VIN2
NC
VINS
VIN3
NC
EN
GND
LTC1477
LBO
GND
DN154 F01
* PRIMARY Li-Ion BATTERY PROTECTION MUST BE
PROVIDED BY AN INDEPENDENT CIRCUIT
** SUMIDA CD54-220
† AVX TPSD107M010R0100
Figure 1. Boost Converter with Short-Circuit Protection and Automatic Load Disconnect
05/97/154_conv
will disconnect the load when the LT1304 is shut down.
Although RSENSE lowers the efficiency near full load,
the additional circuitry has essentially no effect on the
excellent light load efficiency of the LT1304.
Short-Circuit Protection at Higher Power
At higher currents, low RDS(ON) transistors and low
voltage current sensing are necessary to maintain
high efficiency and manageable power dissipation.
The LTC1153 circuit breaker IC drives an external high
side N-channel FET and will turn off the FET when the
voltage across a current sense resistor exceeds 100mV.
Trip delay and reset times can be adjusted with external
components.
The LT1270 boost converter can generate 2A at 12V
from a 5V input (Figure 3). Protection against a shorted
output is provided with an LTC1153 programmed to trip
at 2.5A. At start-up, the LTC1153 drives the gate of Q1
through a filter (R1 and C1). This limits dV/dt at the
output, controlling the inrush current to the capacitive
load. When the LTC1153 senses a voltage drop across
RSENSE exceeding 100mV, the gate of Q1 is grounded
through diode D1. The FET will remain off for a period
determined by the capacitor tied to the CT pin. With
this reset period set longer than the trip delay (60μs
max) and the turn-on time of Q1 (~R1 C1), the average
output current will be much lower than the peak current;
this keeps power dissipation of the FET and the load
at a safe level. An open collector at the STATUS pin of
LTC1153 indicates the state of the circuit breaker. The
output can be disconnected by setting the IN pin low.
The LTC1153 operates from 4.5V to 18V and is available
in an 8-lead PDIP or SO.
L1
22μH
D1
+
+
100μF
Q3
2N3906
2 CELLS
100μF
1M
VIN
ON
1M
OFF
SW
FB
RSENSE
3.9Ω
Q2
2N3906
499k
LT1304
SHDN
GND
Q1
5V
150mA
R1
1M
154k
R2
10k
DN154 F02
D1: MOTOROLA MBRS0520LT1
L1: SUMIDA CD54-220
Q1: SILICONIX Si6433DQ
Figure 2. Current Limited Boost Converter with Load Disconnect
55μH*
5V
+
RSENSE
0.04Ω
STATUS ON/OFF
MBR735
(12V)
+
470μF
1000μF
51k
5
VIN
VSW 4
LT1270
2
FB
VC
GND
3
1
10.72k
1%
IN
VS
CT
DS
LTC1153
1.24k
1%
STATUS
R1
100k
GND
1μF
*PULSE ENGINEERING
PE-92116
Q1
IRLZ34
G
0.47μF
Z5U
1k
D1
1N4148
SHDN
R2
100k
C1
0.22μF
12V
+
12V
2A
47μF
16V
DN154 F03
Figure 3. 2A/12V Step-Up Regulator with Circuit Breaker (Post Regulator),
Breaker Status Feedback and Ramped Output
Data Sheet Download
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