LTM4656 datasheet(10/28 Pages) AD

10 / 28 page

LTM4656/LTM4656-1

Rev. 0

For more information www.analog.com

IN-LINE PROTECTION SECTION

ReferringtotheBlockDiagram,theinputvoltageisapplied

to VIN. The power MOSFET MIN is controlled by a charge

pump high side drive that is slowly turned on after the

SHDN pin is either pulled up to VIN or driven from an open

collectordriveratedtoVIN.TheUVpinhasaninternal100k

resistor to VIN that can be used with an external resistor

to ground to set a UVLO trip point for VIN. This is valuable

to limit turn-on to a specific input voltage level. When

both the SHDN and UV pin thresholds are met, then the

gate voltage begins to ramp up at a control rate and the

MIN source pin will follow. Gate turn-on time is ~10ms.

The RSENSE in series with this path monitors the current

going to the boost converter. The LTM4656 monitors the

voltage drop between the SENSE1 and BVINpinstoprotect

against overcurrent faults. An internal amplifier limits the

voltage across the internal 4mΩ current sense resistor

to 50mV. This is reduced to 25mV in a severe fault when

BVIN is below 2V. In this fault condition, a timer is started

inversely proportional to MOSFET stress. Before the timer

expires,theFLTpinpullslowtowarnofanimpendingpower

down. If the condition persists, the MOSFET is turned off,

and restarts after a cooldown period. BVIN needs at least

100µF capacitance to service proper current limit level to

eliminate any overtemperature timeout oscillations.

FAULT TIMER SECTION

A 0.01µF capacitor is connected internal to the TMR pin

and ground to set the times for early fault warning, fault

turn-off, and cooldown periods. This capacitor is selected

to assure the MIN MOSFET is turn-off fast enough. The

TMRchargingcurrentincreaseslinearlyfrom8µAwithVDS

< 0.5V to 120µA with VDS = 40V. VDS is inferred from the

drop across VIN and SENSE1. See Figure 1. The current

charging up this TMR pin during fault conditions depends

on the voltage difference across MIN MOSFET between

the VIN and SENSE1 pins. This increase in TMR current is

to assure a faster turn off during an overcurrent fault with

substantial voltage across the MIN MOSFET. This turn-off

time is correlated with the MIN MOSFET SOA capability to

APPLICATIONS INFORMATION

assure the MOSFET can handle this power dissipation for

that period of time. When TMR reaches 1.275V, the FLT

pin pulls low to indicate the detection of a fault condition.

If the condition persists, the pass transistor turns off when

TMRreachesthethresholdof1.375V.A2µAcurrentsource

then continues to pull the TMR up. When TMR reaches

4.3V, the 2µA current reverses direction and starts to pull

the TMR pin low. When TMR reaches the retry threshold

of 0.5V, the GATE pin pulls high turning back on the pass

transistor. See overcurrent fault diagram in Figure 2.

Figure 1. Overcurrent TMR Current

vs VIN – SENSE1 Voltage

VIN – SENSE1 (V)

–260

–220

–160

–120

–80

–40

4656 F01

TMR = 1V

Figure 2. Overcurrent Fault Time with 0.01µF

tFLT

= 90µs

TOTAL FAULT TIMER = tFLT + tWARNING

tFLT = 221µs

tWARNING

= 20µs

tWARNING

= 8µs

VTMR(V)

VDS = 12V

ITMR = 35µA

CTMR = 0.01µF

1.375

1.275

0.50

TIME

4656 F02

VDS = 28V

ITMR = 120µA

CTMR = 0.01µF

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