ISL62386 Datasheet, PDF(13/20 Page) Intersil Corporation – High-Efficiency, Quad Output System Power Supply Controller for Notebook Computers

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ISL62386 Datasheet, PDF (13/20 Pages) Intersil Corporation – High-Efficiency, Quad Output System Power Supply Controller for Notebook Computers

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ISL62386

1.5ms

tSOFT-START

VOUT

VCC and LDO5

2.75ms

PGOOD Delay

PGOOD

FIGURE 24. SOFT-START SEQUENCE FOR ONE SMPS

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 ISL62386

includes 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 enabled after SMPS2 is in regulation

Float SMPS2 enabled after SMPS1 is in regulation

VCC

The VCC nominal operation voltage is 5V. If EN1, EN2 and

LDO3EN are all logic low, the VCC start-up voltage is 3.6V

when VIN is applied on ISL62386. As described before, the

LDO5 5V LDO is held off until any of the three enable signals

(EN1, EN2 or LDO3EN) is pulled high. When LDO5 is above

the 4.2V VCC POR threshold, VCC will switchover to LDO5.

After VIN is applied, the VCC start-up 3.6V voltage can be used

as the logic high signal of any of EN1, EN2 and LDO3EN to

enable LDO5 if there is no other power supply on the board.

MOSFET Gate-Drive Outputs LGATE and UGATE

The ISL62386 has 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 ISL62386 has integrated

boot diodes connected from the LDO5 pins to BOOT pins.

tLGFUGR

tUGFLGR

UGATE

LGATE

50%

FIGURE 25. LGATE AND UGATE DEAD-TIME

Diode Emulation

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

ISL62386. If forced high, the ISL62386 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 ISL62386 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

conducts positive inductor current, the phase voltage will be

negative with respect to the GND and PGND pins.

Conversely, when the low-side MOSFET conducts negative

FN6831.0

February 4, 2009

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