A8603KESTR-R datasheet(20/42 Pages) ALLEGRO

20 / 42 page

Multiple-Output Regulator for

Automotive LCD Displays

A8603

Allegro MicroSystems, LLC

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

The A8603 contains an integrated DMOS switch and PWM con-

troller to drive a boost converter. The input voltage, VIN, (3.3 V

nominal) is boosted to an intermediate voltage, VOUT, which is

the lowest voltage required to keep all outputs within regula-

Boost Controller

tion. The final output voltage is decided by the regulator, which

requires the highest boost voltage. This is illustrated in Figure 7.

AVDD

VGL

VGH2

VGH3

-12

-8

-6

-4

-2

10 12 14 16 18 20 22 24 26 28 30

-10

Boost Voltage = highest of:

-VGL + 3.5 V

AVDD + 2.1 V

VGH/2 + 2.9 V

(or VGH/3 + 3.7 V)

Figure 7: Boost Voltage Requirement with Respect to VGL, AVDD and VGH

For example: assume the output requirements for a certain LCD

panel are: VAVDD = 10 V, VVGH = 18 V and VVGL = –7 V, then:

• AVDD (LDO):

VOUT ≥ VAVDD + 2.1 (V) = 12.1 V

• VGH (2× Charge Pump):

VOUT ≥ VVGH / 2 + 2.9 (V) = 11.9 V

• VGL (Inverted Charge Pump):

VOUT ≥ – VVGL + 3.5 (V) = 10.5 V

In this example, AVDD has the highest requirement, so the boost

output voltage will be regulated at a VOUT = 12.1 V approxi-

mately. However, if VVGH were increased to 24 V, it would

require higher voltage, and then the boost converter would

increase the boost output voltage to 14.9 V to satisfy the 2×

charge pump. This leads to higher voltage drop across the linear

regulator for AVDD, and hence higher power loss. In such case, it

is worthwhile to consider the option of 3× charge pump for VGH.

• VGH (2× Charge Pump):

VOUT ≥ VVGH / 2 + 2.9 (V) = 14.9 V

• VGH (3× Charge Pump):

VOUT ≥ VVGH / 3 + 3.7 (V) = 11.7 V

So by using 3× charge pump for VGH, the boost voltage is

reduced to 12.1 V (as dominated by AVDD). This results in lower

power loss and hence better system efficiency.

A block diagram of the A8603 boost controller circuit is shown in

Figure 8. Typical values for external COMP components are RZ

= 511 Ω and CZ = 0.22 µF. Note that the boost stage simply pro-

vides an intermediate voltage. The actual output voltages (AVDD,

VGL, VGH) are controlled by linear regulators and charge

pumps, which contain their own internal compensation.

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