Detailed Description

The MAX8563/MAX8564/MAX8564A triple and dual

LDO controllers allow flexible and inexpensive voltage

conversion by controlling the gate of an external

n-MOSFET in a source-follower configuration. The

MAX8563/MAX8564/MAX8564A consist of multiple

identical LDO controllers. Each LDO controller features

an enable input (EN_) and a power-OK output (POK_).

The MAX8563/MAX8564/MAX8564A also include a 0.5V

reference, an internal regulator, and an undervoltage

lockout (UVLO). The transconductance amplifier mea-

sures the feedback voltage on FB_ and compares it to

an internal 0.5V reference connected to the positive

input. If the voltage on FB_ is lower than 0.5V, the cur-

rent output on the gate-drive output DRV_ is increased.

If the voltage on FB_ is higher than 0.5V, the current out-

put on the gate-drive output is decreased.

The MAX8563/MAX8564/MAX8564A bias current

(typ), the MAX8563/MAX8564/MAX8564A assume that

the supply and reference voltages are too low and acti-

vate the UVLO circuitry. During UVLO, the internal regu-

lator (VL) and the internal bandgap reference are forced

off, DRV_ is pulled to GND, and POK_ is pulled low.

be above the UVLO threshold. After UVLO indicates that

bandgap reference, and the bias currents are activated.

If EN_ is logic-high after the internal reference and bias

currents are activated, then the corresponding DRV_ out-

put initiates operation in soft-start mode. Once the voltage

on FB_ reaches 94% of the regulation threshold, the full

output current of the LDO controller is permitted.

When an LDO is activated, the respective DRV_ is pulled

up from GND with a typical soft-start current of DRV soft-

start. The soft-start current limits the slew of the output

voltage and limits the initial spike of current that the drain

of the external n-MOSFET receives. The size of the com-

3). This output voltage slew rate is equal to (DRV_soft-

soft-start current.

Input Voltage (Drain Voltage of the

External n-MOSFET)

The minimum input voltage to the drain of the n-MOSFET

is a function of the desired output voltage and the

dropout voltage of the n-MOSFET. Details on calculating

this value are covered in the Power MOSFET Selection

section.

The maximum input voltage to the drain of the n-MOSFET

is a function of the breakdown voltage and the thermal

conditions during operation. The breakdown voltage from

drain to source is normally provided in the MOSFET data

sheet. The theoretical maximum input voltage is the set

output voltage plus the breakdown voltage. The thermal

constraint is usually the largest concern when discussing

maximum input voltage. Details on calculating this value

are covered in the Power MOSFET Selection section. The

MOSFET package and thermal relief on the board are

the largest contributors to removing heat from the

n-MOSFET. Since output voltage is normally set and

maximum output current is fixed, the input voltage

becomes the only variable that determines the maxi-

mum power dissipated. Thus, the maximum input volt-

age is limited by the power capability of the n-MOSFET,

if it is less than the breakdown voltage, which is most

often the case. Ensure input capacitors handle the

maximum input voltage.

before the input to the drain of the n-MOSFET, the

MAX8563/MAX8564/MAX8564A drive DRV_ high but the

80% of the regulation voltage, the MAX8563/MAX8564/

MAX8564A assume that an output short-circuit fault is

present and shut down that regulator. To avoid this error

the middle node connected to the respective EN_ (see

Figure 4). Use the following equations to calculate the

resistor values.

07

.

__

>

+

⎛

⎝⎜

⎞

⎠⎟

×

−

R

RR

VV

V

E

ED

DD

IN

IN

±1%, Ultra-Low Output Voltage, Dual and Triple

Linear n-FET Controllers

_______________________________________________________________________________________

9

DRV_

MAX8563

MAX8564

MAX8564A

OUT1

Q1

Figure 3. Soft-Start and Compensation Schematic