LTC4216IDE#PBF Datasheet LTC4216IDE#PBF, LTC4216IMS#PBF, LTC4216IDE#TRPBF | P10-P12

LTC4216

4216fa

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applicaTions inForMaTion

switch when the TIMER pin voltage exceeds its threshold.

The timer period for C1 to charge up to the TIMER pin

threshold, V

TMR(TH)

(1.253V), is given by:

TIMER

1 253 1

.•

(1)

For example, if C1 = 10nF, t

TIMER

= 6.2ms.

FB Glitch Filtering

The FB pin is used to monitor the output voltage of the

external MOSFET through a resistive divider. Any tran

sients on the FB pin due to the output low spikes will

pull RESET

low. To prevent RESET from generating an

unwanted system reset, the FB comparator has a glitch

filter to ride out these glitches. The filter time is 20µs for

large transients (greater than 150mV) and up to 100µs

for small transients. The relationship between glitch filter

time and the FB pin transient voltage or FB overdrive is

shown in Figure 1.

FB pin voltage rises above 0.6V, the FB comparator output

goes low and a new timing cycle starts. After a complete

timing cycle at time point 6, RESET is pulled high by the

external pull-up resistor, R5. The timer period given by

Equation (1) sets the power-good delay for RESET going

high. If the FB pin voltage stays above 0.6V for less than

a timing cycle at time point 4, the RESET output remains

low. Any overcurrent fault detected by the electronic cir

cuit breaker or F

AULT pin driven low externally during the

timing cycle, will also pull the TIMER pin low and RESET

output remains low.

When the device enters an undervoltage lockout condition

or the ON pin voltage drops below 0.4V, RESET is pulled

low, ignoring the FB pin voltage.

Figure 2. Output Voltage Monitor Block Diagram

Figure 3. Output Voltage Monitor

Waveforms in Normal Operation

Output Voltage Monitor

As shown in Figure 2, the output voltage is monitored

through a resistive divider, R3 and R4, connected at the

FB pin, and a FB comparator with 0.6V threshold.

The normal operation of the output voltage monitor after a

start-up cycle is shown in Figure 3. At time point 1, when the

FB pin voltage falls below 0.6V, the FB comparator output

goes high. RESET is pulled low by an internal N-channel

switch after a glitch filter delay at time point 2. When the

Figure 1. FB Comparator Glitch Filter Time vs FB Overdrive

FB OVERDRIVE (mV)

100

120

140

160120

4216 F01

40 80 200

GLITCH FILTER TIME (µs)

= 25°C

–

LOGIC

TIMER

LTC4216**

SENSEP

0.6V

SENSE

RESET

SENSEN GATE

–

LOAD

**ADDITIONAL DETAILS

OMITTED FOR CLARITY

RESET

µP

OUT

4216 F02

1 2

OUT

TMR(TH)

< 0.6V

> 0.6V

< 0.6V V

> 0.6V

TIMER

RESET

GLITCH FILTER DELAY

3 4 5 6

POWER-GOOD

DELAY

2µA

4216 F03

LTC4216

4216fa

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Electronic Circuit Breaker

The LTC4216 features an electronic circuit breaker function

that protects the external MOSFET against short-circuits or

excessive load current conditions on the supply. An external

sense resistor connected between SENSEP and SENSEN

pins is used to measure the load current. If the voltage

across the sense resistor exceeds the circuit breaker trip

threshold of 25mV for more than a fault filter delay, the

gate of the MOSFET is pulled low, turning it off.

The fault filter delay is determined by a capacitor, C3, con

nected between

the FILTER pin and ground as in Equation

(2). The FILTER pin sources 60µA pull-up current when the

sense voltage across the sense resistor exceeds 25mV.

Otherwise, it pulls down with 2.4µA. When the FILTER

pin voltage exceeds V

FILT(TH)

threshold (1.253V), there

is an internal 20µs delay before the GATE pulls low and

the FAULT pin will be pulled low. If no FILTER capacitor

is used, the filter fault delay defaults to 20µs. The circuit

breaker response time or fault filter delay with the FILTER

capacitor, C3, is given by:

CB TRIP()

.•

+µ

1 253 3

(2)

The FILTER capacitor, C3, should be chosen so that the

fault filter delay is not too short to trip the circuit breaker

as the MOSFET current charges up a large output load

capacitance in analog current limit during power-up. It

also should not be too long to exceed the safe operating

area (SOA) of the external MOSFET.

Intermittent overloads may exceed the current limit as in

Figure 5, but if the duration is sufficiently short, the FILTER

pin voltage may not reach the V

FILT(TH)

threshold and the

device will not shut off. To handle this situation, the FILTER

discharges with 2.4µA whenever voltage across the sense

resistor is below 25mV. Any intermittent overload with

an aggregate duty cycle of more than 4% will eventually

trip the circuit breaker. Figure 6 shows the circuit breaker

response time in seconds normalized to 1µF as given by

Equation (3). The asymmetric charging and discharging

of FILTER is a fair gauge of MOSFET heating.

1 253

(/ )

(•)– .

µ=

(3)

Following a circuit breaker trip, the device is latched-off

and FAULT is pulled low until the fault latch is cleared by

pulling the ON pin low (< 0.4V) for at least 100µs. The

FILTER pin is pulled low by an internal N-channel switch

to discharge the capacitor quickly when the ON pin volt

age falls below 0.4V and pulls down with 2.4µA when the

ON pin voltage rises above 0.8V to initiate a new start-up

cycle. The new timing cycle will not start until the FILTER

pin voltage is below 0.2V. The electronic circuit breaker

is disabled during the first timing cycle upon start-up and

any short-circuit faults will be ignored.

Figure 4. A Continuous Fault Timing

Figure 5. Multiple Intermittent Overcurrent Condition

FILTER

CIRCUIT BREAKER TRIPS

A B

1.253V

NORMAL

MODE

FAULT

MODE

2.4µA

60µA

4216 F04

LOAD

FILTER

GATE

CIRCUIT

BREAKER

TRIPS

1.253V

FAULT

A2 B2 A3 B3

25mV/R

SENSE

60µA

2.4µA

LTC4216

4216fa

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Analog Current Limiting

In addition to an electronic circuit breaker, the LTC4216

has included a novel analog current limit (ACL) amplifier

that does not require an external compensation capacitor

at the GATE pin. The amplifier’s stability is compensated

by the large gate input capacitance (C

ISS

) of the external

MOSFET used. These MOSFETs usually have C

ISS

≥ 1nF.

However, if the MOSFET’s gate input capacitance (C

ISS

)

is too small for loop stability, then connect an external

capacitor between the GATE pin and ground to increase

the total gate capacitance to ≥ 1nF. As given by Equation

(4), the MOSFET current, I

ACL

, is limited to the analog

current limit voltage, ΔV

ACL(TH)

, 40mV typical, across

the sense resistor, R

SENSE

, connected between SENSEP

and SENSEN pins.

ACL

ACLTH

SENSE

∆

()

(4)

The ΔV

ACL(TH)

threshold is 1.6 times higher than the

ΔV

CB(TH)

threshold (25mV typical) to provide dual level cur-

rent sensing. When the ACL amplifier servos the MOSFET

current

at ΔV

ACL(TH)

across the sense resistor, it exceeds

ΔV

CB(TH)

threshold causing the FILTER pin to charge C3

with 60µA pull-up. If the condition persists long enough

for C3 to reach the V

FILT(TH)

threshold (1.253V), GATE is

pulled low and FAULT latched low.

If the voltage across the sense resistor is greater than

ΔV

ACL(TH)

during an overload condition, the ACL amplifier

will servo GATE downwards in an attempt to control the

MOSFET current. Since the GATE pin voltage overdrives

the MOSFET in normal operation, the ACL amplifier needs

time to discharge the GATE to the threshold of the MOSFET

for gate regulation. For mild overload, the ACL amplifier

can control the MOSFET current, but in the event of a

severe overload, the MOSFET current may overshoot as

the MOSFET has large GATE overdrive initially. The GATE

is quickly discharged to ground followed by the ACL ampli

fier taking control. For applications that require very fast

analog

current limit recovery from the

GATE undershoot as

it discharges, connect a series resistor, R

, with an exter-

nal capacitor,

, at the GATE pin as shown in Figure 17.

The value of R

should be between 10Ω and 100Ω for

optimum performance.

Soft-Start

The LTC4216 features a soft-start function that controls

the di/dt of the inrush current during power-up. As large

output load capacitors are commonly used in low voltage

applications, the normal inrush can be large enough to

glitch the load supply. With the soft-start function, the

gate of the external MOSFET is allowed to turn on very

gradually to control the inrush current flowing into the

load capacitor without causing a supply glitch.

With an external capacitor, C2, connected between the SS

pin and ground, the GATE is servoed by the ACL amplifier

to track the rate of SS ramp-up during power-up. There

are two slopes in the SS ramp-up profile: 10µA current

source pull-up for a normal ramp rate; and 1µA current

source pull-up for a slower ramp rate. Both the SS ramp

rates are given as follows:

Normal SS Ramp Rate:

SS NOM()

µ10

(5)

Slower SS Ramp Rate:

SS SLOW()

µ1

(6)

Figure 6. Circuit Breaker Filter

Response for Intermittent Overload

OVERLOAD DUTY CYCLE, D (%)

0 20 40 60 80 100

NORMALIZED RESPONSE TIME (s/µF)

0.1

0.01

4216 F06

t/C3(s/µF) = 1.253/[(60 • D) – 2.4]

applicaTions inForMaTion

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P24 P25-P26

LTC4216IDE#PBF

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Manufacturer:

Analog Devices Inc.

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