ISL6442IAZ View Datasheet(PDF) - Renesas Electronics

Part Name

Description

Manufacturer

ISL6442IAZ

Dual (180° Out-of-Phase) PWM and Linear Controller

Renesas Electronics 

ISL6442

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

0

5

10

15

20

25

30

35

40

RT (k)

FIGURE 10. FREQUENCY vs RT RESISTOR

Output Regulation

Figure 11 shows the generic feedback resistor circuit for any of

the three VOUT’s; the VOUT is divided down to equal the

reference. All three use a 0.6V internal reference (check the

“Electrical Specifications” Table on page 4 for the exact

reference value at 24V). The RUP is connected to the VOUT;

the RLOW to GND; the common point goes to the FB pin.

VOUT

RUP

FB

COMP

EA

RLOW

0.6V

FIGURE 11. OUTPUT REGULATION

VOUT must be greater than 0.6V and 2 resistors are needed,

and their accuracy directly affect the regulator tolerance.

FB = VOUT  R-----U----P-R----+L----O-R---W--L---O-----W---

(EQ. 4)

Use Equation 5 to choose the resistor values. RUP is part of

the compensation network for the switchers, and should be

selected to be compatible; 1k to 5k is a good starting value.

Find FB from the “Electrical Specifications” Table on page 5

(for the right condition), plug in the desired value for VOUT, and

solve for RLOW.

RLOW = -V--F--O--B--U----T---R--–--U---F--P--B--

(EQ. 5)

The maximum duty cycle of the ISL6442 approaches 100% at

low frequency, but falls off at higher frequency; see the

“Electrical Specifications” Table on page 5. In addition, there is

a minimum UGATE pulse width, in order to properly sense

overcurrent. The two switchers are 180° out of phase.

The linear output voltage is restricted to approximately a 1V to

4V range. VIN3 should be equal or less than VCC (in order to be

sure that LCDR can turn off the PNP). Note that the linear output

is off until the rising POR trips; it does not have a soft-start ramp,

and it does NOT shut off, unless VCC goes back below the

falling POR trip point. It is suggested that using one of the

switcher outputs as the input to the linear allows it to be ramped

and enabled/disabled with that switcher.

Protection Mechanisms

OCP - (Function independent for both PWM). The overcurrent

function protects the PWM Converter from a shorted output by

using the upper MOSFET’s ON-resistance, rDS(ON) to monitor

the current. This method enhances the converter’s efficiency and

reduces cost by eliminating a current sensing resistor. The

overcurrent function cycles the soft-start function in a hiccup

mode to provide fault protection. A resistor connected to the drain

of the upper MOSFET and OCSET pin programs the overcurrent

trip level. The PHASE node voltage will be compared against the

voltage on the OCSET pin, while the upper MOSFET is on. A

current (typically 110µA) is pulled from the OCSET pin to establish

the OCSET voltage. If PHASE is lower than OCSET while the

upper MOSFET is on then an overcurrent condition is detected for

that clock cycle. The upper gate pulse is immediately terminated,

and a counter is incremented. If an overcurrent condition is

detected for 32 consecutive clock cycles, and the circuit is not in

soft-start, the ISL6442 enters into the soft-start hiccup mode.

During hiccup, the external capacitor on the SS/EN pin is

discharged, then released and a soft-start cycle is initiated. During

soft-start, pulse termination current limiting is enabled, but the 32-

cycle hiccup counter is held in reset until soft-start is completed.

Figure 12 shows an example of the hiccup mode. As the

SS2/EN2 is pulled below the enable trip point, VOUT2 shuts

off, and the voltage goes to GND, at which time the output

current goes to zero. As SS2/EN2 rises above the enable trip

point, the output tries to turn on, the current spikes up, and

then is held constant (by the pulse-termination current limiting);

the output voltage rises, but not up to the desired value. When

the SS2/EN2 ramp reaches ~3V, the cycle repeats, and can

continue indefinitely. If the short-circuit is removed, the output

will ramp up with the next soft-start, and normal operation will

resume. VOUT1 and SS1/EN1 will independently function the

same way.

GND>

VOUT2 (2V/DIV)

0A>

IOUT2 (2A/DIV)

GND>

SS2/EN2 (1V/DIV)

FIGURE 12. OVERCURRENT PROTECTION

FN9204 Rev 2.00

Oct 31, 2008

Page 10 of 17

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