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ManualsBrandsMAXIM INTEGRATED ManualsPMICsPMIC - voltage regulators - DC DC switch controllers QFN 16

MAX17595/MAX17596/MAX17597

Peak-Current-Mode Controllers for

Flyback and Boost Regulators

13Maxim Integrated

Figure 2. Programming EN/UVLO and OVI

Startup Operation

The MAX17595 is optimized for implementing an offline

single-switch flyback converter and has a 20V V

UVLO

wake-up level with hysteresis of 15V (min). In offline

applications, a simple cost-effective RC startup circuit is

used. When the input DC voltage is applied, the startup

resistor (R

START

) charges the startup capacitor (C

START

causing the voltage at the V

pin to increase towards the

wake-up V

UVLO threshold (20V typ). During this time,

the MAX17595 draws a low startup current of 20FA (typ)

through R

START

. When the voltage at V

reaches the

wake-up V

UVLO threshold, the MAX17595 commenc-

es switching and control operations. In this condition, the

MAX17595 draws 2mA (typ) current from C

START

for its

internal operation. In addition, the gate-drive current is

also drawn from C

START

, which is a function of the gate

charge of the external MOSFET used and switching fre-

quency. Since this total current cannot be supported by

the current through R

START

, the voltage on C

START

starts to drop. When suitably configured, as shown

in Figure 3, the external MOSFET is switched by the

NDRV pin and the flyback converter generates pulses

on bias winding NB. The soft-start period of the con-

verter should be programmed so the bias winding

pulses sustain the voltage on C

START

before it falls

below 7V, thus allowing continued operation. The large

hysteresis of the MAX17595 allows for a small startup

capacitor (C

START

). The low startup current (20FA typ)

allows the use of a large startup resistor (R

START

), thus

reducing power dissipation at higher DC bus voltages.

START

might need to be implemented as equal, multiple

resistors in series (R

IN1

, R

IN2

, and R

IN3

) to share the

applied high DC voltage in offline applications so

that the voltage across each resistor is limited to its

maximum continuous operating voltage rating. R

START

and C

START

can be calculated as:

+× ×





= µ

××



+×





VDRV IN SS

START

SS G SW

C I t 0.1

C 0.75 F

t Qf

0.04

where I

is the supply current drawn at the V

pin in

mA, Q

is the gate charge of the external MOSFET used

in nC, f

is the switching frequency of the converter in

Hz, and t

is the soft-start time programmed for the fly-

back converter in ms. C

VDRV

is a cummulative capacitor

used in V

DRV

node in μF. See the Programming Soft-Start

of Flyback/Boost Converter (SS) section.

( )

START

V 10 50

−×





where C

START

is the startup capacitor in FF.

For designs that cannot accept power dissipation in the

startup resistors at high DC input voltages in offline appli-

cations, the startup circuit can be set up with a current

source instead of a startup resistor as shown in Figure 4.

The startup capacitor (C

START

) can be calculated using

the above equation:

Resistors R

SUM

and R

ISRC

can be calculated as:

START

SUM

BEQ1

ISRC

= W

The V

UVLO wake-up threshold of the MAX17596/

MAX17597 is set to 4.1V (typ) with a 200mV hyster esis,

optimized for low-voltage DC-DC applications down to

4.5V. For applications where the input DC voltage is low

enough (e.g., 4.5V to 5.5V DC) that the power loss

incurred to supply the operating current of the MAX17596/

MAX17597 can be tolerated, the V

pin is directly

connected to the DC input, as shown in Figure 5. In the

case of higher DC input voltages (e.g., 16V to 32V DC),

a startup circuit, such as that shown in Figure 6, can be

used to minimize power dissipation. In this startup

OVI

DC1

SUM

DC2

DC3

EN/UVLO

OVI

MAX17595

MAX17596

MAX17597