Datasheet

MAX17595/MAX17596/MAX17597
Peak-Current-Mode Controllers for
Flyback and Boost Regulators
12Maxim Integrated
n-Channel MOSFET Gate Driver (NDRV)
The devices offer a built-in gate driver for driving an
external n-channel MOSFET. The NDRV pin can source/
sink peak currents in excess of 900mA/1500mA.
Maximum Duty Cycle
The MAX17595/MAX17596 operate at a maximum duty
cycle of 49%. The MAX17597 offers a maximum duty
cycle of 94% to implement flyback and boost converters
involving large input-to-output voltage ratios in DC-DC
applications. Slope compensation is necessary for sta-
ble operation of peak-current-mode controlled convert-
ers such as the MAX17595/MAX17596/MAX17597, in
addition to the loop compensation required for small
signal stability. The MAX17595/MAX17596/MAX17597
implement a SLOPE pin for this purpose. See the Slope
Compensation section for more details.
Soft-Start (SS)
The devices implement soft-start operation for the fly-
back/boost regulators. A capacitor connected to the SS
pin programs the soft-start period. The soft-start feature
reduces input inrush current during startup. When the
voltage on the SLOPE pin is more than 0.2V, the refer-
ence to the internal error amplifier is ramped up from
0V to 1.21V in a linear manner, as programmed by the
soft-start capacitor. See the Programming Soft-Start (SS)
(SS) section.
Switching Frequency Selection (RT)
The ICs’ switching frequency is programmable between
100kHz and 1MHz with resistor R
RT
connected between
RT and SGND. Use the following formula to determine the
appropriate value of R
RT
needed to generate the desired
output-switching frequency (f
SW
):
=
10
RT
SW
10
R
f
where f
SW
is the desired switching frequency.
Frequency Dithering for
Spread-Spectrum Applications (Low EMI)
The switching frequency of the converter can be
dithered in a range of Q10% by connecting a capaci-
tor from DITHER/SYNC to SGND, and a resistor from
DITHER to RT. Spread-spectrum modulation technique
spreads the energy of switching frequency and its har-
monics over a wider band while reducing their peaks,
helping to meet stringent EMI goals.
Applications Information
Startup Voltage and Input Overvoltage
Protection Setting (EN/UVLO, OVI)
The devices’ EN/UVLO pin serves as an enable/disable
input, as well as an accurate programmable input UVLO
pin. The devices do not commence startup operation
unless the EN/UVLO pin voltage exceeds 1.21V (typ).
The devices turn off if the EN/UVLO pin voltage falls
below 1.15V (typ). A resistor-divider from the input DC
bus to ground can be used to divide down and apply a
fraction of the input DC voltage (V
DC
) to the EN/UVLO
pin. The values of the resistor-divider can be selected so
the EN/UVLO pin voltage exceeds the 1.23V (typ) turn-
on threshold at the desired input DC bus voltage. The
same resistor-divider can be modified with an additional
resistor (R
OVI
) to implement input overvoltage protec-
tion in addition to the EN/UVLO functionality as shown
in Figure 2. When voltage at the OVI pin exceeds
1.21V (typ), the devices stop switching and resume
switching opera tions only if voltage at the OVI pin falls
below 1.15V (typ). For given values of startup DC input
voltage (V
START
) and input overvoltage-protection
voltage (V
OVI
), the resistor values for the divider can
be calculated as fol lows, assuming a 24.9kI resistor
for R
OVI
:
OVI
EN OVI
START
V
R R 1k
V

=×−


I
where R
OVI
is in kI, while V
START
and V
OVI
are in volts.
START
SUM OVI EN
V
R R R 1k
1.21

= +× −




I
where R
EN
and
R
OVI
are in kI, while V
START
is in volts.
In universal AC input applications, R
SUM
may need to be
implemented as equal resistors in series (R
DC1
, R
DC2
,
and R
DC
) so that voltage across each resistor is limited
to its maximum operation voltage.
= = =
SUM
DC1 DC2 DC3
R
RR R k
3
I
For low-voltage DC-DC applications based on the
MAX17596/MAX17597, a single resistor can be used in
the place of R
SUM
, as the voltage across it is approxi-
mately 40V.