DN402 - Electronic Circuit Breaker in Small DFN Package Eliminates Sense Resistor

advertisement
Electronic Circuit Breaker in Small DFN Package Eliminates Sense
Resistor – Design Note 402
SH Lim
Introduction
Traditionally, an Electronic Circuit Breaker (ECB) comprises a MOSFET, a MOSFET controller and a current sense
resistor. The LTC® 4213 does away with the sense resistor
by using the RDS(ON) of the external MOSFET. The result
is a simple, small solution that offers a significant low
insertion loss advantage at low operating load voltage.
The LTC4213 features two circuit breaking responses to
varying overload conditions with three selectable trip
thresholds and a high side drive for an external N-channel MOSFET switch.
Overcurrent Protection
The SENSEP and SENSEN pins monitor the load current via the RDS(ON) of the external MOSFET and serve
as inputs to two internal comparators—SLOWCOMP
and FASTCOMP—with trip points at VCB and VCB(FAST),
respectively. The circuit breaker trips when an overcurrent fault causes a substantial voltage drop across the
MOSFET. An overload current exceeding VCB/RDS(ON)
causes SLOWCOMP to trip the circuit breaker after a 16µs
delay. In the event of a severe overload or short-circuit
current exceeding VCB(FAST)/RDS(ON), the FASTCOMP
trips the circuit breaker within 1µs, protecting both the
MOSFET and the load.
Both of the comparators have a common mode input
voltage range from ground to VCC + 0.2V. This allows
the circuit breaker to operate as the load supply turns
on from 0V.
Flexible Overcurrent Setting
The LTC4213 has an ISEL pin to select one of these three
overcurrent settings:
ISEL at GND, VCB = 25mV and VCB(FAST) = 100mV
ISEL left open, VCB = 50mV and VCB(FAST) = 175mV
ISEL at VCC, VCB = 100mV and VCB(FAST) = 325mV
Overvoltage Protection
The LTC4213 can provide load overvoltage protection
(OVP) above the bias supply. When VSENSEP > VCC + 0.7V
for 65µs, an internal OVP circuit activates with the GATE
pin pulling low and the external MOSFET turning off. The
OVP circuit protects the system from an incorrect plug-in
event where the VIN load supply is much higher than the
VCC bias voltage.
Typical Electronic Circuit Breaker (ECB)
Application
Figure 1 shows the LTC4213 in a dual supply ECB application. An input bypass capacitor is recommended to
prevent transient spikes when the VIN supply powers-up
or the ECB responds to overcurrent conditions. Figure 2
shows a normal power-up sequence. The LTC4213 exits
reset mode once the VCC pin is above the internal under
voltage lockout threshold and the ON pin rises above
0.8V (see trace 1 in Figure 2). After an internal 60µs debounce cycle, the GATE pin capacitance is charged up
from ground by an internal 100µA current source (see
trace 2). As the GATE pin and the gate of MOSFET charges
up, the external MOSFET turns on when VGATE exceeds
the MOSFET’s threshold. The circuit breaker is armed
when VGATE exceeds ΔVGSARM, a voltage at which the
external MOSFET is deemed fully enhanced and RDS(ON)
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology
Corporation. All other trademarks are the property of their respective owners.
Q1
SI4864DY
VIN
1.25V
CIN
100µF
VBIAS
3.3V
OFF ON
+
CLOAD
100µF
VCC SENSEP GATE SENSEN
C1
0.1µF
LTC4213
ON
GND
ISEL
+
VOUT
1.25V
3.5A
VCC
R4
10k
READY
DN402 F01
Figure 1. The LTC4213 in an Electronic Circuit
Breaker Application
10/06/402
minimized. Then, 50µs after the circuit breaker is armed,
the READY pin goes high (see trace 3) and signals the
system to power up VIN. Trace 4 shows the related VOUT
waveform when VIN powers up. In order to not trip the
circuit breaker during startup, the load current must be
lower than VCB/RSENSE. If needed, the ISEL pin can be
stepped dynamically for a higher overcurrent threshold
at startup and a lower threshold when the load current
has stabilized.
Accurate ECB with Sense Resistor
The RDS(ON) voltage drop sensing method trades the
circuit breaker accuracy for system simplicity. The
majority of sensing inaccuracy is due to the external
MOSFET’s RDS(ON) varied by operating temperature and
under different VGS bias condition. The MOSFET vendors
also do not specify the RDS(ON) distribution tightly due
to manufacturing variation. If an external tight tolerance
resistor is employed for current sensing instead, the
LTC4213 reveals its ±10% circuit breaker accuracy. Figure
(1)
VON
1V/DIV
3 shows a tolerable RSENSE resistor voltage drop and the
LTC4213 is used for accurate ECB applications.
High Side Switch for N-Channel Logic Level
MOSFET
Logic level N-Channel MOSFET applications usually
requires a minimum gate drive voltage of 4.5V. Figure 4
shows the LTC4213 in a High Side Switch application.
The LTC4213’s internal charge pump boosts the GATE
above the logic level gate drive requirement and ensures
the MOSFET is fully enhanced for VCC ≥ 3V. The typical
gate drive versus bias supply voltage curve is shown in
Figure 5.
Conclusion
The LTC4213 is a small package, No RSENSE Electronic
Circuit Breaker that is ideally suited for low voltage applications with low MOSFET insertion loss. It includes
selectable dual current level and dual response time circuit
breaker functions. The circuit breaker has wide operating
input common-mode-range from ground to VCC.
VIN
3V TO 6V
Q1
Si4420BDY
+
10µF
(2)
VGATE
5V/DIV
R4
10k
VOUT
3V TO 6V
5A
+
CLOAD
VCC
SENSEP
SENSEN
GATE
READY
LTC4213
(3)
VREADY
2V/DIV
OFF ON
ON
GND
ISEL
DN402 F04
(4)
VOUT ≈ VIN
1V/DIV
VIN POWERS UP
0.1ms/DIV
DN402 F02
Figure 4. High Side Switch for Logic
Level N-Channel MOSFET, VCC > 3V
Figure 2. Normal Power-Up Sequence
8.0
VIN
1.9V
+
Q1
Si4420BDY
VOUT
1.9V
5A
+
CIN
100µF
SENSEP
VBIAS
5V
VCC
OFF ON
ON
CLOAD
100µF
GATE
SENSEN
VIN
R4
10k
LTC4213
7.5
VGATE – VSENSEN (V)
RSENSE
7mΩ
7.0
6.5
6.0
5.5
5.0
4.5
READY
GND
ISEL
DN402 F03
4.0
3.0
3.5
4.0
5.0
5.5
4.5
BIAS SUPPLY VOLTAGE (V)
6.0
DN402 F05
Figure 3. Accurate ECB with High Side Sense Resistor
Figure 5. Gate Drive Voltage vs Bias Supply Voltage
Data Sheet Download
http://www.linear.com
For applications help,
call (408) 432-1900, Ext. 2452
Linear Technology Corporation
dn402f LT 1006 305K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
Similar pages