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Micrel, Inc. MIC2310 Evaluation Board

July 2008

M9999-070808-A

Output Turn-On and GATE Control (dI

/dt

or dV

GATE

/dt)

With the input supply voltage applied, the MIC2310 is

enabled by applying a logic HIGH input voltage signal on

the ENABLE control pin. Upon the application of an

ENABLE LOW-to-HIGH transition after the Power-on

Reset delay (t

POR

), or at the end of t

POR

if ENABLE is

already HIGH, a nominal start-up commences where the

/dt-controlled inrush current (by dI

/dt = 17.6x10

-3

SLEW

/ (R

SENSE

×C

SLEW

)) is permitted to exceed the I

POC

threshold for t

POC

, until there is sufficient charge stored on

the load capacitor as evidenced by the output load voltage

profile. Note that the secondary overcurrent detection

threshold (I

SOC

) is set externally at the controller’s S[1:0]

pins. Once the inrush current exceeds the I

SOC

threshold,

the circuit breaker trips without delay and the MIC2310

controller shuts down the output. If the inrush current

profile does not cause either of the OC detection circuits to

trip the circuit breaker and assert the I_FLT digital output,

the controller will assert the PWRGD digital output when

the output load voltage is higher than the controller’s V

PGH

threshold voltage and the V

of the external MOSFET is

higher than the controller’s V

GSPGH

threshold voltage. Due

to the low R

DS(ON)

of the external MOSFET, the output load

voltage rises with the GATE voltage as the V

of the

MOSFET reaches its threshold voltage. Once the output

load voltage stabilizes near the V

supply voltage, the V

of the external MOSFET increases above its threshold

voltage and eventually exceeds V

GSPGH

. The PWRGD

output asserts to signal that the external MOSFET is fully

enhanced and ready for the application of the full load.

An alternate mode for the output load charging can be

achieved by use of the dV

GATE

/dt controlled profile. This

mode is realized by leaving the CSLEW pin OPEN

(floating JP8) and connecting a capacitor from GATE to

AGND.

Circuit Breaker Function

The MIC2310 employs a dual-level circuit breaker function

to protect against excessive load currents. The dual-level

circuit breaker “trips” when either

1) the load current remains above the primary

overcurrent threshold (V

CBP

) yet below the

secondary, or fast-trip, threshold current limit for a

programmed fault time, t

POC

, where t

POC

POCSENSE

+ C

PRIMARY

∗ (V

PRIL

PRI

), or

2) the voltage monitored across the sense resistor

exceeds the 2-bit user-programmed secondary

overcurrent threshold, V

CBS

. In the latter case, the

GATE output is immediately disabled to shut down

the output. The value for the C

PRIMARY

capacitor

(C5) can be chosen to set the duration of t

POC

MIC2310 Highlighted Features

Fast Load Discharge Circuit (Optional)

The MIC2310 features a DISCH output pin that provides a

signal to drive an external transistor to quickly discharge

the bulk capacitance at the output. In general, these

transistors are used either as a solitary switch with other

passive components or in tandem with an SCR and

external passive components for an extremely fast output

load discharge. For the MIC2310 EVB, jumper JP5

provides an option for a very fast load discharge through

the external crowbar circuit consisting of two transistors,

an SCR, and several passive components. Shorting JP5 to

ground will disable the discharge circuitry and allows the

load to discharge much slower, but the HW_FLT will

normally assert in this case due to the ∆V between the

SOURCE and GATE exceeding the V

GSPGH

threshold.

Adaptable Current Limit Threshold

The MIC2310 will maintain a constant power product (V·A)

over the entire operating voltage range of 10.8V-13.2V.

This is achieved by dynamically adjusting the current-limit

threshold based on the voltage level monitored by the

LOADSENSE and GNDSENSE pins and fed back to the

MIC2310 current limit threshold control circuitry. For

example, in a 240-VA (240W) application, when the

voltage level falls to 11.0V, the MIC2310 controller would

allow for close to 22A of current to flow before tripping the

circuit breaker. Conversely, if the voltage level rises to

13.0V, the MIC2310 controller allows approximately 18.5A

of current to flow before tripping the circuit breaker.

Auto-Retry or Latch Off Fault Modes

The MIC2310 has an optional configuration for overcurrent

fault responses, either employing an output auto-retry or

latch off mode. With jumper JP6 left open, the MIC2310 is

configured for auto-retry of the output upon an overcurrent

fault event. Capacitor C6 sets the auto-retry period and is

determined by

RETRY

= C

RETRY

* (V

RETRYH

RETRYUP

)

With JP6 shorted to ground, the MIC2310 will latch the

output upon an overcurrent fault event and remains

latched OFF unless the ENABLE input is toggled HIGH-to-

LOW-to-HIGH as defined by t

ENLPW

(from the AC Electrical

Characteristics Table in the MIC2310 datasheet) or the

supply voltage is turned OFF then ON.

Digital Signal Output Indicators

There are three MIC2310 digital outputs that are used to

drive LEDs to indicate several occurrences:

1) The “Power-is Good” (PWRGD or /PWRGD) will

be asserted when the output voltage level is

higher than the controller’s V

PGH

threshold.

2) The overcurrent fault signal (I_FLT or /I_FLT) will

assert when there is a primary overcurrent

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