LT3759 datasheet(14/44 Pages) LINER

14 / 44 page

LT8710

8710f

For more information www.linear.com/LT8710

Note that if the INTVEE LDO is in UVLO and SS > 1.8V

(typical), then the voltage reference at the positive input

of EA3 is 916mV (typical), resulting in limiting the output

current to about half of its set limit.

OPERATION – RESET CONDITIONS

The LT8710 has three reset cases. When the part is in

reset, the SS pin is pulled low and both power switches,

MN and MP, are forced off. Once all of the reset conditions

are gone, the part is allowed to begin a soft-start sequence

andswitchingcancommence.Eachofthefollowingevents

can cause the LT8710 to be in reset:

1. UVLO

a. The greater of VIN and BIAS is < 4.5V (maximum)

b. INTVCC < 4V (typical)

c. EN/FBIN < 1.7V (typical) at first power-up

2. Overcurrent sensed by IMON > 1.38V (typical)

3. Die Temperature > 175°C

OPERATION – POWER SWITCH CONTROL

The main power switch is the external NFET (MN in Block

Diagram) and the synchronous power switch is the ex-

ternal PFET (MP in Block Diagram). The two switches

are never on at the same time, and there is a non-overlap

time of ~140ns and ~90ns on the rising and falling edges

respectively (see Electrical Characteristics) to prevent

cross conduction. Figure 4 below shows the BG and TG

(BIAS–TG) signals:

Light Load Current (MODE Pin)

The MODE pin can be used to tell the LT8710 to operate

in forced CCM regardless of load current, or operate in

DCM at light loads.

• MODE < 1.175V (typical) = Forced CCM or FCM

• MODE > 1.224V (typical) = DCM or Pulse-Skipping

The forced continuous mode (FCM) allows the inductor

current to reverse directions without any switches being

forced off. At very light load currents, the inductor cur-

rent will swing positive and negative as the appropriate

average current is delivered to the output. There are some

exceptions that negate the MODE pin and force the part

to operate in DCM at light loads:

1. The INTVEE LDO is in UVLO (BIAS – INTVEE < 3.42V

typical).

2. SS < 1.8V (typical).

3. The part is in a reset condition.

When the LT8710 is in discontinuous mode (DCM), syn-

chronous switch MP is held off whenever MP’s current

falls near 0 current (less than 2.8mV (typical) across

RSENSE2). This is to prevent current draw from the output

and/or feeding current to the input supply. Under very

light loads, the current comparator A7, may also remain

tripped for several cycles (i.e. skipping pulses). Since MP

is held off during the skipped pulses, the inductor current

will not reverse.

OPERATION – C/10 AND POWER GOOD (FLAG PIN)

The FLAG pin is an open-drain pin that functions as an ac-

tive high C/10 and power good pin. The FLAG pin changes

states 100µs (typical) after the internal comparators tell

the FLAG pin to change states to reject glitches or tran-

sient events.

operaTion

Figure 4. Synchronous Switching

8710 F04

140ns

90ns

LT3759 Datasheet(PDF) 14 Page - Linear Technology

LT3759 Datasheet(HTML) 14 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Datasheet Download

Link URL