ISL62381 Datasheet, PDF(15/23 Page) Intersil Corporation – High-Efficiency, Quad or Triple-Output System Power Supply Controller for Notebook Computers

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ISL62381 Datasheet, PDF (15/23 Pages) Intersil Corporation – High-Efficiency, Quad or Triple-Output System Power Supply Controller for Notebook Computers

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ISL62381, ISL62382, ISL62383

1.5ms

tSOFTSTART

2.75ms

PGOOD Delay

VOUT

VCC and LDO5

PGOOD

FIGURE 24. SOFT-START SEQUENCE FOR ONE SMPS

The PGOOD pin indicates when the converter is capable of

supplying regulated voltage. It is an undefined impedance if

VIN is not above the rising POR threshold or below the POR

falling threshold. When a fault is detected, the ISL62381,

ISL62382 and ISL62383 will turn on the open-drain NMOS,

which will pull PGOOD low with a nominal impedance of 63Î©

or 95Î©. This will flag the system that one of the output

voltages is out of regulation.

Separate enable pins allow for full soft-start sequencing.

Because low shutdown quiescent current is necessary to

prolong battery life in notebook applications, the LDO5 5V

LDO is held off until any of the three enable signals (EN1,

EN2 or LDO3EN) is pulled high. Soft-start of all outputs will

only start until after LDO5 is above the 4.2V POR threshold.

In addition to user-programmable sequencing, the

ISL62381, ISL62382 and ISL62383 include a

pre-programmed sequential SMPS soft-start feature. Table 1

shows the SMPS enable truth table.

TABLE 1. SMPS ENABLE SEQUENCE LOGIC

EN1

EN2

START-UP SEQUENCE

Both SMPS outputs OFF simultaneously

Float Both SMPS outputs OFF simultaneously

Float

Both SMPS outputs OFF simultaneously

Float

Both SMPS outputs OFF simultaneously

SMPS1 OFF, SMPS2 ON

SMPS1 ON, SMPS2 OFF

Both SMPS outputs ON simultaneously

Float

1 SMPS1 enables after SMPS2 is in regulation

Float SMPS2 enables after SMPS1 is in regulation

MOSFET Gate-Drive Outputs LGATE and UGATE

The ISL62381, ISL62382 and ISL62383 have internal

gate-drivers for the high-side and low-side N-Channel

MOSFETs. The low-side gate-drivers are optimized for low

duty-cycle applications where the low-side MOSFET

conduction losses are dominant, requiring a low r DS(ON)

MOSFET. The LGATE pull-down resistance is small in order

to clamp the gate of the MOSFET below the VGS(th) at

turnoff. The current transient through the gate at turn-off can

be considerable because the gate charge of a low r DS(ON)

MOSFET can be large. Adaptive shoot-through protection

prevents a gate-driver output from turning on until the

opposite gate-driver output has fallen below approximately

1V. The dead-time shown in Figure 25 is extended by the

additional period that the falling gate voltage stays above the

1V threshold. The typical dead-time is 21ns. The high-side

gate-driver output voltage is measured across the UGATE

and PHASE pins while the low-side gate-driver output

voltage is measured across the LGATE and PGND pins. The

power for the LGATE gate-driver is sourced directly from the

LDO5 pin. The power for the UGATE gate-driver is sourced

from a âbootâ capacitor connected across the BOOT and

PHASE pins. The boot capacitor is charged from the 5V

LDO5 supply through a âboot diodeâ each time the low-side

MOSFET turns on, pulling the PHASE pin low. The

ISL62381, ISL62382 and ISL62383 have integrated boot

diodes connected from the LDO5 pins to BOOT pins.

tLGFUGR

UGATE

LGATE

50%

tUGFLGR

FIGURE 25. LGATE AND UGATE DEAD-TIME

Diode Emulation

FCCM is a logic input that controls the power state of the

ISL62381, ISL62382 and ISL62383. If forced high, the

ISL62381, ISL62382 and ISL62383 will operate in forced

continuous-conduction-mode (CCM) over the entire load

range. This will produce the best transient response to all

load conditions, but will have increased light-load power

loss. If FCCM is forced low, the ISL62381, ISL62382 and

ISL62383 will automatically operate in diode-emulation-

mode (DEM) at light load to optimize efficiency in the entire

load range. The transition is automatically achieved by

detecting the load current and turning off LGATE when the

inductor current reaches 0A.

Positive-going inductor current flows from either the source

of the high-side MOSFET, or the drain of the low-side

MOSFET. Negative-going inductor current flows into the

drain of the low-side MOSFET. When the low-side MOSFET

FN6665.4

August 7, 2008

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