ISL6534CVZ datasheet(21/29 Pages) RENESAS

21 / 29 page

ISL6534

FN9134 Rev 2.00

Page 21 of 29

Nov 18, 2005

Figure 20 shows the upper gate drive supplied by a direct

connection to VCC12. This option should only be used in

converter systems where the main input voltage is +5 VDC

or less. The peak upper gate-to-source voltage is

approximately VCC12 less the input supply. For +5V main

power and +12 VDC for the VIN bias, the gate-to-source

voltage of Q1 is 7V. A logic-level MOSFET may be a good

choice for Q1 (again, check the max gate voltage ratings)

and a logic-level MOSFET can be used for Q2 if its absolute

gate-to-source voltage rating exceeds the maximum voltage

applied to VCC12.

Bootstrap Trade-offs

Bootstrapping to 12V requires that the upper FET have a

maximum gate-source rating of greater than 12V. Since the

LGATE output is sourced from the VCC12 supply in all

cases, the lower FET must also have the high rating. So this

may rule out using some 20V breakdown FETs that have

gate ratings of 12V or less.

Figure 19 shows the diode DBOOT and bootstrap capacitor

CBOOT. A small capacitor (~1µF; not shown) is sometimes

used as a local decoupling cap to GND; it should be placed

near the anode of the diode to GND.

The anode of the diode is shown tied to VCC12, but it can

also connect to VCC (even in the shunt regulator mode in

some cases) or to VIN or to another appropriate supply. If

the shunt voltage is used for bootstrapping, it does increase

the current used in the shunt; check that the shunt voltage is

not affected; if it is, a lower value of shunt resistor may be

necessary.

Figure 20 shows the direct hookup; the advantage is that two

components (DBOOT and CBOOT) are not needed; a

possible disadvantage is that the VCC12 may not be the

optimum voltage for efficiency (perhaps a bootstrap

diode/capacitor to 5V would be better, for example).

The PHASE node is not brought into the ISL6534, so there is

no way to reference the gate voltage to it, as is often done in

other regulators. The considerations for the BOOT2 pin are

identical to BOOT1; but since they may have different VIN,

VOUT, FETs, etc., the preferred solution for each output may

be different for any given system.

The voltage required on VBOOT (Bootstrap Voltage; the

diode anode) depends primarily on the upper NFET rDS(ON)

and Vth. A high voltage makes the rDS(ON) as low as

possible, which should help the overall efficiency; however,

the high voltage makes the switching power in the gate

driver higher, which lowers the efficiency. So the net overall

effect is a trade-off between the two. At the other extreme, a

low voltage must be at least as high as the FET threshold

voltage, plus a few volts of overdrive, in order to turn on the

NFET hard enough to source the maximum load current. So

the rDS(ON) is not as low, hurting the efficiency, but the gate

driver power is lower, which helps the efficiency.

Since the gate driver power is a function of (voltage)2, the

theoretical optimum VBOOT voltage is to make it only high

enough to turn on the NFET to handle the maximum load.

However, since there are usually only a few available power

supplies to choose from, the user often must compromise.

And sometimes the only supply available is the same one

used for VIN, which may be good for one term, but not as

good for the other.

The size of the bootstrap capacitor can be chosen by using

the following equations:

The last equation plugs in some typical values: N = 1; QG is

33nC, VIN is 12V, VGS is 11V, Vmax = 1V. In this example,

CBOOT  0.051µF. This value is often rounded up to 0.1µF

as a starting value. The bootstrap capacitors usually need to

be rated at 16V, to handle the typical 12V boot.

In general, as the number of FETs or the size of the FETs

increases (which usually makes QG larger) or if VIN or the

bootstrap supply (if not VIN) increases (for example, from 5V

to 12V), these all require that CBOOT become larger.

FIGURE 20. UPPER GATE DRIVE - DIRECT VCC12 DRIVE

OPTION

+12V

PGND

LGATE

UGATE

PHASE

BOOT

VCC12

+5V OR LESS

NOTE:

VG-S  VCC12 - 5V

NOTE:

VG-S  VCC12

VCC12

D2 (OPTIONAL)

ISL6534

GND

PGND

(CHANNEL

1 OR 2)

CBOOT

QGATE

V

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



QGATE

NQG VIN



VGS

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

CBOOT

QGATE

V

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

NQG VIN



VGS V



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

13312



11 0.7



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

0.051

F



where

N is the number of upper FETs

QG is the total gate charge per upper FET

VIN is the input voltage

VGS is the gate-source voltage (usually VIN - diode drop)

V is the change in boot voltage before and immediately

after the transfer of charge; typically 0.7V to 1.0V

and

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