LINEAR TECHNOLOGY LTC4213 Technical data

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Electronic Circuit Breaker in Small DFN Package Eliminates Sense
Resistor – Design Note 402

SH Lim

Introduction

Traditionally, an Electronic Circuit Breaker (ECB) com­prises a MOSFE T, a MOSFET controller and a curren t sense

®

resistor. The LTC
by using the R

4213 does away with the sense resistor

of the external MOSFET. The result

DS(ON)

is a simple, small solution that offers a signifi cant 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 R

of the external MOSFET and serve

DS(ON)

as inputs to two internal comparators—SLOWCOMP
and FASTCOMP—with trip points at V

CB

and V

CB(FAST)

,
respectively. The circuit breaker trips when an overcur­rent fault causes a substantial voltage drop across the
MOSFET. An overload current exceeding V

CB/RDS(ON)

causes SLOWCOMP to trip the cir cuit breaker after a 16µs
delay. In the event of a severe overload or short-circuit
current exceeding V

CB(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 V

+ 0.2V. This allows

CC

the circuit breaker to operate as the load supply turns
on from 0V.

Flexible Overcurrent Setting

The LTC4213 has an I

pin to select one of these three

SEL

overcurrent settings:

at GND, VCB = 25mV and V

I

SEL

I

left open, VCB = 50mV and V

SEL

I

at V

, V

SEL

CC

CB

= 100mV and V

CB(FAST)

CB(FAST)

CB(FAST)

= 100mV

= 175mV

= 325mV

Overvoltage Protection

The LTC4213 can provide load overvoltage protection
(OVP) above the bias supply. When V

SENSEP

> V

CC

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 V

bias voltage.

V

CC

load supply is much higher than the

IN

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 V

supply powers-up

IN

or the ECB responds to overcurrent conditions. Figure 2
shows a normal power-up sequence. The LTC4213 exits
reset mode once the V

pin is above the internal under

CC

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 V

GATE

exceeds
the MOSFET’s threshold. The circuit breaker is armed
when V

exceeds ΔV

GATE

, a voltage at which the

GSARM

external MOSFET is deemed fully enhanced and R

, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technolog
Corporation. All other trademarks are the property of their respective owners.

Q1

1.25V

V

BIAS

3.3V

OFF ON

V

IN

+

C

IN

100µF

V

C1

0.1µF

ON READY

SI4864DY

C

LOAD

100µF

SENSENSENSEP

LTC4213

GND

GATE

I

SEL

DN402 F01

CC

10k

V

OUT

1.25V

+

3.5A

V

CC

R4

+ 0.7V

DS(ON)

10/06/402

Figure 1. The LTC4213 in an Electronic Circuit
Breaker Application

minimized. Then, 50µs after the circuit breaker is armed,
the READY pin goes high (see trace 3) and signals the
system to power up V
waveform when V

. Trace 4 shows the related V

IN

powers up. In order to not trip the

IN

OUT

circuit breaker during startup, the load current must be
lower than V

CB/RSENSE

. If needed, the I

pin can be

SEL

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 R

voltage drop sensing method trades the

DS(ON)

circuit breaker accuracy for system simplicity. The
majority of sensing inaccuracy is due to the external
MOSFET’s R
under different V
also do not specify the R

varied by operating temperature and

DS(ON)

bias condition. The MOSFET vendors

GS

distribution tightly due

DS(ON)

to manufacturing variation. If an external tight tolerance
resistor is employed for current sensing instead, the
LTC4213 reveals it s ±10% circuit bre aker accuracy. Figure

(1)

V

ON

1V/DIV

(2)

V

GATE

5V/DIV

(3)

V

READY

2V/DIV

(4)

≈ V

V

OUT

IN

1V/DIV

Figure 2. Normal Power-Up Sequence

V

IN

1.9V

+

C

IN

100µF

V

BIAS

V

CC

5V

ONOFF

ON

R

GND

SENSE

7mΩ

SENSEN

LTC4213

VIN POWERS UP

0.1ms/DIV

Si4420BDY

GATESENSEP

I

SEL

Q1

READY

DN402 F03

DN402 F02

V

OUT

1.9V
5A

+

C

LOAD

100µF

V

IN

R4
10k

3 shows a tolerable R

resistor voltage drop and the

SENSE

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 V

≥ 3V. The typical

CC

gate drive versus bias supply voltage curve is shown in
Figure 5.

Conclusion

The LTC4213 is a small package, No R

SENSE

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 V

Q1

3V TO 6V

V

IN

+

R

4

10µF

10k

SENSEP

CC

READY

LTC4213

ONOFF

ON

GND

Figure 4. High Side Switch for Logic
Level N-Channel MOSFET, V

8.0

7.5

7.0

(V)

6.5

SENSEN

6.0

– V

5.5

GATE

V

5.0

4.5

4.0

3.5 4.0 5.0

3.0

4.5

BIAS SUPPLY VOLTAGE (V)

Si4420BDY

SENSEN GATEV

I

SEL

DN402 F04

> 3V

CC

5.5

DN402 F05

.

CC

+

C

6.0

V

OUT

3V TO 6V
5A

LOAD

Figure 3. Accurate ECB with High Side Sense Resistor

Data Sheet Download

http://www.linear.com

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

●

[phone]

FAX: [phone] ● www.linear.com

Figure 5. Gate Drive Voltage vs Bias Supply Voltage

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call [phone], Ext. 2452

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