ISL8225M
commonly used conditions. Otherwise, use the following procedures
to finish the EN/FF design:
Functional Description
Initialization
1. A resistor divider from V to GND is recommended to set the
IN
EN/FF voltage between 1.25V to 5.0V. The resistor divider
ratio is recommended to be between 3/1 to 4/1 (as shown in
Figure 21) with a resistor divider at 7.15kΩ/2.05kΩ.
Initially, the Power-On Reset (POR) circuits continuously monitor
bias voltages (V ) and voltage at the EN/FF pin. The POR
CC
function initiates soft-start operation 384 clock cycles
after: (1) the EN pin voltage is pulled above 0.8V, (2) all input
supplies exceed their POR thresholds, and (3) the PLL locking
time expires. The Enable pin can be used as a voltage monitor
and to set the desired hysteresis, with an internal 30µA sinking
current going through an external resistor divider. The sinking
current is disengaged after the system is enabled. This feature is
specially designed for applications that require higher input rail
POR for better under-voltage protection. For example, in 12V
2. Check EN turn-on hysteresis (Recommend V
> 0.3V) :
(EQ. 4)
EN_HYS
–5
V
= N • R • 3x10
UP
EN – HYS
where:
• R is the top resistor of the resistor divider
UP
• N is the total number of the EN/FF pins tied to the resistor divider
3. Set the maximum current flowing through the top pull-up
applications, R = 53.6kΩ and R
turn-on threshold (V
EN_RTH
= 5.23kΩ sets the
UP
DOWN
) to 10.6V and the turn-off threshold
).
resistor R to below 7mA (considering EN/FF is pulled to
UP
ground (V
= 0)). Refer to Figure 23; a 3.01kΩ/1kΩ
EN/FF
(V
) to 9V, with 1.6V hysteresis (V
EN_FTH EN_HYS
resistor is used to allow for the input voltage from 5V to 20V
operation. In addition, the maximum current flowing through
R5 is 6.6mA (<7mA).
During shutdown or fault conditions, soft-start is quickly reset,
and the gate driver immediately changes state (<100ns) when
input drops below POR.
4. If the EN/FF is controlled by system EN signal instead of the
input voltage, we recommend setting the fixed EN/FF voltage
to about 1/3.5 of the input voltage. If the input voltage is 12V,
a 3.3V system EN signal can be tied to EN/FF pin directly.
Enable and Voltage Feed-forward
Voltage applied to the EN/FF pin is fed to adjust the sawtooth
amplitude of the channel. Sawtooth amplitude is set to 1.25 times
the corresponding FF voltage when the module is enabled. This
configuration helps maintain a constant gain. This configuration
also helps maintain input voltage to achieve optimum loop response
over a wide input voltage range.
5. If the input voltage is below 5.5V, it is recommended to have
EN/FF voltage >1.5V to have better stability. The input voltage
can be directly tied to the VCC pin to disable the internal LDO.
6. A 1nF capacitor is recommended on the EN/FF pin to avoid
the noise injecting into the feed-forward loop.
A 384-cycle delay is added after the system reaches its rising
POR and prior to soft-start. The RC timing at the FF pin should be
small enough to ensure that the input bus reaches its static state
and that the internal ramp circuitry stabilizes before soft-start. A
large RC could cause the internal ramp amplitude not to
synchronize with the input bus voltage during output start-up or
when recovering from faults. A 1nF capacitor is recommended as
a starting value for typical applications.
Thermal Considerations
The ISL8225M QFN package offers typical junction to ambient
thermal resistance θ of approximately 10°C/W at natural
JA
convection (~5.8°C/W at 400LFM) with a typical 4-layer PCB.
Therefore, use Equation 5 to estimate the module junction
temperature:
(EQ. 5)
T
= P × Θ + T
junction
jA ambient
In a multi-module system, with the EN pins are wired together, all
modules can immediately turn off, at one time, when a fault
condition occurs in one or more modules. A fault pulls the EN pin
low, disabling all modules, and does not create current bounce;
thus, no single channel is overstressed when a fault occurs.
where:
• T
is the module internal maximum temperature (°C)
is the system ambient temperature (°C)
junction
• T
ambient
• P is the total power loss of the module package (W)
Because the EN pins are pulled down under fault conditions, the
• θ is the thermal resistance of module junction to ambient
JA
pull-up resistor (R ) should be scaled to sink no more than 7mA
UP
current from the EN pin. Essentially, the EN pins cannot be
directly connected to VCC.
If the calculated temperature, T
design target, the extra cooling scheme is required. Please refer
to “Current Derating” on page 24 for adding air flow.
, is over the required
junction
VIN
R
² V
V
EN_HYS
UP
= --------------------------------------------------------------
EN_REF
– V
R
V
= ----------------------------
R
UP
DOWN
I
V
R
EN_HYS
UP
EN_FTH
EN_REF
384
0.8V
CLOCK
EN
SOFT-START
= V
– V
CYCLES
EN_FTH
EN_RTH
EN_HYS
ON/OFF
R
DOWN
I
= 30µA
EN_HYS
OV, OT, OC, AND PLL LOCKING FAULTS
FIGURE 26. SIMPLIFIED ENABLE AND VOLTAGE FEED-FORWARD CIRCUIT
FN7822.1
January 31, 2013
20
INTERSIL [ Intersil ]
