ISL78225 Datasheet, PDF (19/21 Pages) Intersil Corporation – 4-Phase Interleaved Boost PWM Controller with Light Load Efficiency Enhancement

ISL78225
channel is scaled and summed together at this pin. An RC
network should be connected between the IOUT pin and GND,
such that the ripple current signal can be filtered out and
converted to a voltage signal to represent the averaged total
input current. The relationship between total input current I_IN and
V_IOUT can be calculated as Equation 10 (see Figure 20 on
page 16 for R_SEN and R_SET positions):
V_IOUT
=
0.75
I_IN
R_{-----S---E---N--}
R_SET
R_IO
UT
(EQ. 10)
When the V_IOUT is higher than 2V for a consecutive 100µs, the
ISL78225 IC will be triggered to shut down. This provides
additional safety for the voltage regulator.
Equation 11 can be used to calculate the value of the resistor R_IOUT
based on the desired OCP level I_{AVG, OCP2}.
R_IOUT
=
-------------²--------------
I_{AVG, OCP2}
(EQ. 11)
The total average overcurrent protection scheme will not be
asserted until the soft-start pin voltage V_SS reaches its clamped
value (approximately 3.5V). During the soft-start time, the system
does not latch-off if per-channel or overall OC limit is reached.
Instead, the individual channel current will run at its pre-set peak
current limit level.
Thermal Protection
The ISL78225 will be disabled if the die junction temperature
reaches a nominal of +160°C. It will recover when the junction
temperature falls below a +15°C hysteresis. The +15°C
hysteresis insures that the device will not be re-enabled until the
junction temperature has dropped to below about +145°C.
Internal 5V LDO Output Current
Limit Derating Curves
ISL78225 contains an internal 5V/200mA LDO, and the input of
LDO (VIN pin) can go as high as 40V. Based on the junction to
ambient thermal resistance R_JA of the package, we need to
guarantee that the maximum junction temperature should be
below +125°C T_MAX. Figure 22 shows the relationship between
maximum allowed LDO output current and input voltage. The curve
is based on +35°C/W thermal resistance R_JA for the package.
Each curve represents different ambient temperature, T_A.
200
T_A = +25°C
180
160
T_A = +50°C
140
T_A = +75°C
120
100
80
60
40
T_A = +100°C
20
0
6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40
V_IN (V)
FIGURE 22. I_LDO(MAX) vs V_IN
Dedicated VREF2 Pin for Input
Voltage Tracking
A second reference input pin, VREF2, is added to the input of the
transconductance amplifier. The ISL78225 internal reference will
automatically change to VREF2 when it is pulled below 1.8V. The
VREF2 pin can be connected to VIN through resistor network to
implement the automatic input voltage tracking function. This
function is very useful under car battery voltage cranking
conditions (such as when the car is parked and the driver is
listening to the stereo), where the full load power is typically not
needed. In this case, the ISL78225 can limit the output power by
allowing the output voltage to track the input voltage. If VREF2 is
not used, the pin should be connected to VCC.
Configurations for Dual IC
Operations
For high power applications, two ISL78225 ICs can be easily
configured to support 8-phase operation. The IC that provides the
CLK_OUT signal is called master IC, and the IC that received the
CLK_OUT signal is called slave IC. Note that the two PWM1
signals are synchronized and the net effect is 4-phase operation
with double the output current.
DRIVE_EN
CLK_OUT
MASTER IC
COMP FB SS
SYSTEM
DRIVE_EN
DRIVE_EN
SYNC
SLAVE IC
COMP FB SS
FIGURE 1. CONFIGURATIONS FOR DUAL IC OPERATION
Figure 1 shows the step-by-step setup as follows:
19
FN7909.0
December 15, 2011