MAX5974A MAXIM [Maxim Integrated Products], MAX5974A Datasheet - Page 19

• Manufacturer Part Number: MAX5974A
• Description: Active-Clamped, Spread-Spectrum, Current-Mode PWM Controllers
• Manufacturer: MAXIM [Maxim Integrated Products]

The frequency foldback threshold can be programmed
from 0 to 20% of the full load current using a resistor from
FFB to GND.
The CS voltage is sampled during the NDRV on-time,
averaged using an internal RC filter, and gained up
by 10 to generate a voltage, V
falls below V
frequency to 1/2 the original value to reduce switching
losses and increase the converter efficiency. The new
switching frequency starts at the beginning of a new
cycle as shown in Figure 5. Calculate the value of R
as follows:
where R
I
triggers frequency foldback, R
sense resistor connected between CS and PGND, and
I
The maximum duty cycle is determined by the lowest
of three voltages: 2V, the voltage at SS (V
voltage (2.43V - V
calculated as:
where V
Figure 5. Entering Frequency Foldback
LOAD(LIGHT)
FFB
is the current sourced from FFB to R
Frequency Foldback for High-Efficiency
MIN
FFB
R
= minimum (2V, V
FFB
FFB
is the resistor between FFB and GND,
is the current at light-load conditions that
AUXDRV
V
, the device folds back the switching
COMP
CSAVG
NDRV
=
V
DCLMP
FFB
10 I
D
MAX
×
LOAD(LIGHT)
). The maximum duty cycle is
=
Light-Load Operation
2.43V
I
Duty-Cycle Clamping
V
FFB
SS
MIN
Active-Clamped, Spread-Spectrum,
CS
CSAVG
, 2.43V - V
t
SW
is the value of the
×
R
. When V
CS
FFB
Current-Mode PWM Controllers
DCLMP
SS
(30FA typ).
), and the
CSAVG
).
t
SW
FFB
x 2
By connecting a resistor between SS and ground, the
voltage at SS can be made to be lower than 2V. V
calculated as follows:
where R
GND, and I
(10FA typ).
To set D
a resistive divider between the supply voltage, DCLMP,
and GND as shown in the Typical Application Circuits.
This feed-forward duty-cycle clamp ensures that the
external n-channel MOSFET is not stressed during sup-
ply transients. V
where R
values shown in the Typical Application Circuits and V
is the input supply voltage.
The internal oscillator can be synchronized to an external
clock by applying the clock to DITHER/SYNC directly. The
external clock frequency can be set anywhere between
1.1x to 2x the internal clock frequency.
MAX
DCLMP1
SS
V
DCLMP
SS-CH
is the resistor connected between SS and
using supply voltage feed-forward, connect
DCLMP
t
SW
and R
is the current sourced from SS to R
V
=
x 2
SS
R
DCLMP1
Oscillator Synchronization
=
DCLMP2
is calculated as follows:
R
R
SS
DCLMP2
×
+
I
SS-CH
R
are the resistive divider
DCLMP2
×
V
S
DCLMP
SS
SS
SS
19
is
S