34704 datasheet(25/54 Pages) FREESCALE

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Analog Integrated Circuit Device Data

Freescale Semiconductor

34704

FUNCTIONAL DESCRIPTION

FUNCTIONAL INTERNAL BLOCK DESCRIPTION

• Operates at a switching frequency equals to FSW2

• Drives integrated low RDS(ON) N-channel power

MOSFETs (NVHV_LC) as its output stage

• It offers load disconnect from the input battery when the

output is off (True Cut-Off)

• The output is ±4% accuracy

• Output voltage is adjustable by means of an external

resistor divider when in voltage regulation mode

• A 240 mV current limit comparator will be used to program/

sense the voltage drop across the current setting resistor

at the bottom of the LED string connected

to the REG8

output when the current regulation mode is selected.

This will be used to program the maximum current flowing

and will regulate it

• The output can be adjusted up or down at 2.5% steps for

a total of 10% on each direction allowing Dynamic Voltage

Scaling

• Maximum output current is adjustable by means of an

external resistor connected to the FB8 pin and then the

output current can be scaled down from the set maximum

in 16 steps through I2C interface

• Uses a bootstrap network with an internal diode to power

its synchronous MOSFET

• All gate drive circuits are supplied from VG.

• Uses integrated compensation

• The output is monitored for over-current and short-circuit

conditions

• The regulator is monitored for over-temperature conditions

• The output is monitored for under-voltage and over-

voltage conditions

Operation Modes

The switchers will be active when:

• VG is in regulation AND

• There is no GrpD shutdown command through the I2C

interface AND

• No faults exist that would cause GrpD to shut down

OVERALL EFFICIENCY ANALYSIS

In battery applications, it is highly recommended to power

every single regulator directly from the battery to obtain full

output capability:

VBAT

V1 (5.0 V)

REG1

VBAT

V2 (2.8 / 3.3 V)

REG2

VBAT

V3 (1.2 V / 1.5 V / 1.8 V)

REG3

VBAT

V4 (1.8 V / 2.5 V)

REG4

VBAT

V5 (3.3 V)

REG5

VBAT

V6 (15 V)

REG6

VBAT

V7 (-7.0 V)

REG7

VBAT

V8 (15 V)

REG8

Figure 5. Overall Efficiency Analysis

Efficiency analysis includes the following losses:

• MOSFET Conduction Losses

• MOSFET Switching Losses (Except for REG7 due to

external MOSFET and board layout dependence)

• MOSFET Gate Charging Losses

• MOSFET Deadtime Losses

• External Diode Losses (Only for REG7)

• Inductor Winding DC Losses

• Inductor Core Losses (Assumed to be 20% of DC Losses

as a rule of thumb)

• Output AC Losses

Efficiency Analysis

In this configuration, all of the regulators are supplied or

powered directly with 3.6 V nominal, battery voltage.

Efficiency was calculated using the maximum allowed

frequency of 1.5 MHz and 1.0 MHz for FSW1 and FSW2,

respectively, in this configuration. As a result, the following

numbers are valid for worst case operation conditions.

The following table shows the detailed analysis for each

regulator with V2 at 3.3 V, V3 at 1.2 V, and V4 at 1.8 V.

34704 Datasheet(PDF) 25 Page - Freescale Semiconductor, Inc

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