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March 19, 2009

down to a lower drive voltage with GVOT can improve the

switching losses seen and maximize system efficiency.

Figure 2 shows that the gate drive voltage optimization is

accomplished via an internal low drop out regulator (LDO)

that regulates the lower gate drive voltage. LVCC is driven to

a lower voltage depending on the GD_SEL pin impedance.

The input and output of this internal regulator are the VCC

and LVCC pins, respectively. Both VCC and LVCC should be

decoupled with a high quality, low ESR ceramic capacitor.

In the 8 Ld SOIC package, the ISL6622B drives the upper

gate to 12V while the lower drive voltage is fixed at 5.75V. The

10 Ld DFN part offers more flexibility: the upper gate can be

driven from 5V to 12V via the UVCC pin, while the lower gate

has a resistor-selectable drive voltage of 5.75V, 6.75V, and

7.75V (typically). This provides the flexibility necessary to

optimize applications involving trade-offs between gate

charge and conduction losses. Table 1 shows the LDO output

(LVCC) level set by GD_SEL pin impedance.

Figure 3 illustrates the internal LDO’s variation with the

average load current plotted over a range of temperatures

shunt the LDO, as shown in Figure 2. The resistor thus

delivers part of the LGATE drive current, leaving less current

going through the internal LDO, elevating the LDO’s output

voltage. Further reduction in RCC’s value can raise the

LVCC voltage further, as desired.

Power-On Reset (POR) Function

During initial start-up, the VCC voltage rise is monitored. Once

the rising VCC voltage exceeds the rising POR threshold,

operation of the driver is enabled and the PWM input signal

takes control of the gate drives. If VCC drops below the POR

falling threshold, operation of the driver is disabled.

Pre-POR Overvoltage Protection

While VCC is below its POR level, the upper gate is held low

and LGATE is connected to the PHASE pin via an internal

10k

Ω (typically) resistor. By connecting the PHASE node to the

gate of the low side MOSFET, the driver offers some passive

protection to the load if the upper MOSFET(s) is or becomes

shorted. If the PHASE node goes higher than the gate

threshold of the lower MOSFET, it results in the progressive

turn-on of the device and the effective clamping of the PHASE

node’s rise. The actual PHASE node clamping level depends

on the lower MOSFET’s electrical characteristics, as well as the

characteristics of the input supply and the path connecting it to

the respective PHASE node.

Internal Bootstrap Device

The ISL6622B features an internal bootstrap Schottky diode.

Simply adding an external capacitor across the BOOT and

PHASE pins completes the bootstrap circuit. The bootstrap

function is also designed to prevent the bootstrap capacitor

from overcharging due to the large negative swing at the

trailing-edge of the PHASE node. This reduces the voltage

stress on the BOOT to PHASE pins.

The bootstrap capacitor must have a maximum voltage rating

well above the maximum voltage intended for UVCC. Its

minimum capacitance value can be estimated from Equation 1:

control MOSFETs. The

ΔV

allowable droop in the rail of the upper gate drive. Select

results are exemplified in Figure 4.

TABLE 1. LDO OPERATION AND OPTIONS

PWM INPUT GD_SEL PIN

LVCC @ 50mA DC LOAD

Don’t Care

Floating

5.75V (Typical; Fixed in SOIC Package)

4.5k

Ω to GND

6.75V(Typical)

GND

7.75V(Typical)

FIGURE 2. GATE VOLTAGE OPTIMIZATION (GVOT) DETAIL

EXTERNAL CIRCUIT

ISL6622B INTERNAL CIRCUIT

VIN

VCC

LVCC

1µF

1µF

SET BY

GD_SEL

GVOT

LDO

RCC = OPTION FOR HIGHER LVCC

RCC

THAN PRE-SET BY GD_SEL

+

-

+

-

>

LGATE

DRIVER

5.0

5.5

6.0

6.5

7.0

7.5

8.0

8.5

9.0

0

20406080

100

AVERAGE LOAD CURRENT (mA)

FIGURE 3. TYPICAL LVCC VARIATION WITH LOAD

GD_SEL TIED TO GND

GD_SEL TIED TO 4.5k

Ω TO GND

GD_SEL FLOATING

+120°C

+40°C

-40°C

C

BOOT_CAP

Q

GATE

ΔV

BOOT_CAP

--------------------------------------

≥

Q

GATE

Q

G1

UVCC

•

V

GS1

------------------------------------

N

Q1

•

=

(EQ. 1)

ISL6622B