Datasheet

MAX17595/MAX17596/MAX17597
Peak-Current-Mode Controllers for
Flyback and Boost Regulators
16Maxim Integrated
trip level (V
CS-PEAK
) of 300mV. Use the following equa-
tion to calculate the value of R
CS
:
CS
MOSFET
300mV
R
I
= I
where I
MOSFET
is the peak current flowing through the
MOSFET.
The devices implement 65ns of leading-edge blank-
ing to ignore leading-edge current spikes. Use a
small RC network for additional filtering of the lead-
ing edge spike on the sense waveform when need-
ed. Set the corner frequency between 10MHz and
20MHz. After the leading-edge blanking time,
the device monitors V
CS
. The switching cycle is termi-
nated within 30ns from V
CS
exceeding 300mV.
The devices offer a runaway current limit scheme that
protects the devices under high-input-voltage short-
circuit conditions when there is insufficient output volt-
age available to restore inductor current built up during
the on period of the flyback/boost converter. Either eight
consecutive occurrences of the peak-current-limit event
or one occurrence of the runaway current limit trigger a
hiccup mode that protects the converter by immediately
suspending switching for a period of time (t
RSTART
).
This allows the overload current to decay due to power
loss in the converter resistances, load, and the output
diode of the flyback/boost converter before soft-start
is attempted again. The runaway current limit is set
at a V
CS-PEAK
of 360mV (typ). The peak-current-limit-
triggered hiccup operation is disabled until the end of
the soft-start period, while the runaway current-limit-
triggered hiccup operation is always enabled.
Programming Slope
Compensation (SLOPE)
The MAX17595/MAX17596 operate at a maximum duty
cycle of 49%. In theory, they do not require slope
compensation to prevent subharmonic instability that
occurs naturally in continuous-conduction mode (CCM)
peak-current-mode-controlled converters operating at
duty cycles greater than 50%. In practice, the MAX17595/
MAX17596 require a minimum amount of slope compen-
sation to provide stable operation. The devices allow the
user to program this default value of slope compensation
simply by leaving the SLOPE pin unconnected. It is rec-
ommended that discontinuous-mode designs also use
this minimum amount of slope compensation to provide
better noise immunity and jitter-free operation.
The MAX17597 flyback/boost converter can be designed
to operate in either discontinuous-conduction mode
(DCM) or to enter into continuous-conduction mode
at a specific load condition for a given DC input
voltage. In continuous-conduction mode, the flyback/
boost converter needs slope compensation to avoid
subharmonic instability that occurs naturally over all
specified load and line conditions in peak-current-mode-
controlled converters operating at duty cycles greater
than 50%. A minimum amount of slope signal is added to
the sensed current signal even for converters operating
below 50% duty to provide stable, jitter-free operation.
The SLOPE pin allows the user to program the necessary
slope compensation by setting the value of the resistor
(R
SLOPE
) connected from the SLOPE pin to ground.
E
SLOPE
S8
Rk
1.55
−
= I
where the slope (S
E
) is expressed in mV/Fs.
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 capacitor from
DITHER/SYNC to SGND, and a resistor from DITHER
to RT as shown in the Typical Operating Circuits. This
results in lower EMI.
A current source at DITHER/SYNC charges capacitor
C
DITHER
to 2V at 50FA. Upon reaching this trip point, it
discharges C
DITHER
to 0.4V at 50FA. The charging and
discharging of the capacitor generates a triangular wave-
form on DITHER/SYNC with peak levels at 0.4V and 2V
and a frequency that is equal to:
=
×
TRI
DITHER
50 A
f
C 3.2V
F
Figure 7. Programming Output Voltage
FB
R
U
R
B
V
OUT
MAX17595
MAX17596
MAX17597