it operates in linear mode. Q1 typically has an input voltage rating of 30V (see Section 14.1 CHARGE PASS TRANSISTOR (Q1)).

However, if Q1 fails as a short, the shunt control mechanism of the IC will still protect the phone and cell from high voltages. The

internal M3 shunt transistor is used to clamp the VIN, CELL and RADIO_B+ power pins to safe values.

13.1.2 Safety Shunt/Crowbar Operation

The LM3655 only activates the over-voltage protection if there is an internal failure of the LM3655 or a failure to the external

components, such as Q1 being shorted. Should one of these failures occur and the cell voltage exceeds a nominal level of 4.35V

(V

mechanism occurs if V

When operating as a shunt regulator (in the linear mode of M3), excessive current may cause M3 power dissipation to exceed

safe limits if uncontrolled. If the M3 current limit of 4 Amps (typical I

fully on in order to shunt the current. When in the safety shunt crowbar mode, the IC will turn off Q1 to stop the charge current

assuming that the external components and the control circuit of the IC are still operational.

13.1.3 Standby Shunt Mode

Another protection feature of the IC is a Standby shunt mode. When there is no charger present M3 is turned on as it would be

in Crowbar mode. Q1 is already turned off because the charger isn’t present. M3 is turned off when there becomes a charger

present. In this condition, the other crowbar modes function as is. Standby shunt mode doesn’t interfere with the fast and thermal

(see Section 13.1.4 Thermal Crowbar Mode and Section 13.1.5 Fast Shunt Operation) crowbar modes.

13.1.4 Thermal Crowbar Mode

Another safety protection feature of the IC is the thermal crowbar protection. As the charge power supply’s current compliance

increases, significant power dissipation occurs and the junction temperature of the IC could approach T

initiation temperature and the power NMOS device M3 will be switched to full conduction mode in order to protect the IC. The

PMOS device M5 is switched off in Crowbar mode to prevent high cell discharge currents. The charge power supply’s current may

or may not open the external discrete fuse. If the fuse survives, the only recovery method from Crowbar1 mode is for the junction

temperature to reduce below T

τ

off Q1 to stop the charge current assuming that the external components and the control circuit of the IC are still operational.

13.1.5 Fast Shunt Operation

The VIN pin voltage is also AC-coupled to the shunt control network to provide a “fast comparator” function that can activate the

shunt transistor quickly in response to a fast-rising transient on the input. This allows the shunt device to be activated prior to the

other means of activation such as exceeding I

DC-level sensitive comparator circuits.

In actual operation it will typically be very unlikely to activate the input shunt without activating the cell-initiated shunt mechanism

because the two voltages are related via M5 conduction.

13.1.6 Shunt Circuit Parametric Specifications

Unless otherwise noted, the following shunt specifications apply over the Normal Temperature Range.

Specification

Test Conditions

Min

Typ

Max

Units

Crowbar Temperature “T

105

120

135

˚C

Crowbar Reset Temperature “T

607590

˚C

Fast Crowbar Current “I

τ

2

4

7.5

A

Fast Crowbar Delay “

τ

1

50

µs

Crowbar1 Sustain Time “

τ

4

8

16

s

Crowbar2 Sustain Time “

τ

32

64

128

ms

Primary Shunt Control Voltage “V

I

4.30

4.25

4.35

4.35

4.40

4.45

V

V

V

100

190

350

mV

Secondary Shunt Control Voltage at V

(V

I

V

4.7

5.2

5.7

V

τ

STBYSHUNT

Delay when CP goes from 1 to 0 before M3

gets turned on.

125

ms

V

STBY_SHUNT

0.1A to 2A into V

IN

0.5

1.1

2.2

V

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