MAX15004A 데이터 시트보기 (PDF) - Maxim Integrated

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MAX15004A

4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers

Maxim Integrated 

MAX15004A/B-MAX15005A/B

Startup Operation/UVLO/ON/OFF

The MAX15004A/B/MAX15005A/B feature two undervolt-

age lockouts (UVLO). The internal UVLO monitors the

VCC-regulator and turns on the converter once VCC rises

above 3.5V. The internal UVLO circuit has about 0.5V

hysteresis to avoid chattering during turn-on.

An external undervoltage lockout can be achieved by

controlling the voltage at the ON/OFF input. The ON/

OFF input threshold is set at 1.23V (rising) with 75mV

hysteresis.

Before any operation can commence, the ON/OFF volt-

age must exceed the 1.23V threshold.

Calculate R1 in Figure 1 by using the following formula:

= R1







VON

VUVLO



− 1



×

R2

where VUVLO is the ON/OFF’s 1.23V rising threshold,

and VON is the desired input startup voltage. Choose an

R2 value in the 100kΩ range. The UVLO circuits keep

the PWM comparator, ILIM comparator, oscillator, and

output driver shut down to reduce current consumption

(see the Functional Diagram). The ON/OFF input can be

used to disable the MAX15004/MAX15005 and reduce

the standby current to less than 20μA.

Soft-Start

The MAX15004/MAX15005 are provided with an

externally adjustable soft-start function, saving a number

of external components. The SS is a 1.228V reference

bypass connection for the MAX15004A/B/MAX15005A/B

and also controls the soft-start period. At startup, after

VIN is applied and the UVLO thresholds are reached, the

device enters soft-start. During soft-start, 15μA is sourced

into the capacitor (CSS) connected from SS to GND

causing the reference voltage to ramp up slowly. The

HICCUP mode of operation is disabled during soft-start.

When VSS reaches 1.228V, the output as well as the

HICCUP mode become fully active. Set the soft-start time

(tSS) using following equation:

t SS

=

1.23(V) × CSS

15 ×10 −6(A)

where tSS is in seconds and CSS is in farads.

The soft-start programmability is important to control

the input inrush current issue and also to avoid the

MAX15004/MAX15005 power supply from going into the

unintentional hiccup during the startup. The required soft-

start time depends on the topology used, current-limit

setting, output capacitance, and the load condition.

4.5V to 40V Input Automotive

Flyback/Boost/SEPIC

Power-Supply Controllers

MAX15004A/B

MAX15005A/B

ON/OFF

1.23V

VIN

R1

R2

Figure 1. Setting the MAX15004A/B/MAX15005A/B

Undervoltage-Lockout Threshold

Oscillator Frequency/External Synchronization

Use an external resistor and capacitor at RTCT to pro-

gram the MAX15004A/B/MAX15005A/B internal oscilla-

tor frequency from 15kHz to 1MHz. The MAX15004A/B

output switching frequency is one-half the programmed

oscillator frequency with a 50% maximum duty-cycle

limit. The MAX15005A/B output switching frequency is

the same as the oscillator frequency. The RC network

connected to RTCT controls both the oscillator frequen-

cy and the maximum duty cycle. The CT capacitor

charges and discharges from (0.1 x VREG5) to (0.55 x

VREG5). It charges through RT and discharges through an

internal trimmed controlled current sink. The maximum

duty cycle is inversely proportional to the discharge time

(tDISCHARGE). See Figure 3a and Figure 3b for a coarse

selection of capacitor values for a given switching frequen-

cy and maximum duty cycle and then use the following

equations to calculate the resistor value to fine-tune the

switching frequency and verify the worst-case maximum

duty cycle.

t CHARGE

=

DMAX

fOSC

RT = t CHARGE

0.7 × CT

t DISCHARGE

=

(1.33

2.25(V) × RT × CT

× 10 −3(A) × RT) − 3.375(V)

fOSC

=

t CHARGE

+

1

t DISCHARGE

where fOSC is the oscillator frequency, RT is the

resistance connected from RTCT to REG5, and CT is

the capacitor connected from RTCT to SGND. For the

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