ISL6521 Datasheet, PDF (7/13 Pages) Intersil Corporation – PWM Buck DC-DC and Triple Linear Power Controller

ISL6521
The OC trip point varies with MOSFET’s r_DS(ON)
temperature variations. To avoid overcurrent tripping in the
normal operating load range, determine the R_OCSET
resistor from the equation above with:
1. The maximum r_DS(ON) at the highest junction temperature.
2. The minimum I_OCSET from the specification table.
3. Determine I_PEAK for I_PEAK > I_OUT(MAX) + (∆I)/2, where
∆I is the output inductor ripple current.
OVERCURRENT TRIP:
VDS > VSET
iD ¥ rDS_(ON) > IOCSET ¥ ROCSET
OCSET
V_IN = +5V
R_OCSET
OC
+
-
OCC
PWM
I_OCSET
40µA
DRIVE
GATE
CONTROL
V_SET +
VCC
UGATE
PHASE
i_D
+
V_DS(ON)
VPHASE = VIN – VDS
VOCSET = VIN – VSET
FIGURE 3. OVERCURRENT DETECTION
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection’.
Output Voltage Selection
The output voltage of the PWM converter can be resistor-
programmed to any level between V_IN and 0.8V. However,
since the value of R_S1 is affecting the values of the rest of
the compensation components, it is advisable its value is
kept between 2kΩ and 5kΩ.
+5V_IN
Q3
V_OUT3
+
C_OUT3
V_OUT4
+
C_OUT4
DRIVE3
FB3
R_S3
R_P3
ISL6521
DRIVE4
FB4
R_S4
R_P4
VOUT
=
0.8
×



1
+
RR-----SP-- _{}
FIGURE 4. ADJUSTING THE OUTPUT VOLTAGE OF ANY OF
THE FOUR REGULATORS (OUTPUTS 3 AND 4
PICTURED)
Output voltage selection on the linear regulators is set by
means of external resistor dividers as shown in Figure 4.
The two resistors used to set the voltage on each of the
three linear regulators have to meet the following criteria:
their value while in a parallel connection has to be less than
5kΩ, or otherwise said, the following relationship has to be
met:
-_RR----_SS-----+^×-----^RR-----^PP- < 5kΩ
To ensure the parallel combination of the feedback resistors
equals a certain chosen value, R_FB, use the following
equations:
R_S
=
V_{-----O----U----T--}
V_FB
×
R_FB
R_P = -_V---^R-_O----^SU----_T-^×----–-^V----_V^F----^BF----_B- , where
V_OUT - the desired output voltage,
V_FB - feedback (reference) voltage, 0.8V.
Application Guidelines
Soft-Start Interval
The soft-start function controls the output voltages rate of rise
to limit the current surge at start-up. The soft-start function is
integrated on the chip and the soft-start interval is fixed.
PWM Controller Feedback Compensation
The PWM controller uses voltage-mode control for output
regulation. This section highlights the design consideration
for a PWM voltage-mode controller. Apply the methods and
considerations only to the PWM controller.
Figure 5 highlights the voltage-mode control loop for a
synchronous-rectified buck converter. The output voltage
(V_OUT) is regulated to the reference voltage level, 0.8V. The
error amplifier (Error Amp) output (V_E/A) is compared with
the oscillator (OSC) triangular wave to provide a pulse-width
modulated (PWM) wave with an amplitude of V_IN at the
PHASE node. The PWM wave is smoothed by the output
filter (L_O and C_O).
The modulator transfer function is the small-signal transfer
function of V_OUT/V_E/A . This function is dominated by a DC
Gain, given by V_IN/V_OSC, and shaped by the output filter,
with a double pole break frequency at F_LC and a zero at
F_ESR .
Modulator Break Frequency Equations
F_LC=
-------------------¹--------------------
2π × L_O × C_O
F_ESR=
--------------------¹---------------------
2π × ESR × C_O
The compensation network consists of the error amplifier
(internal to the ISL6521) and the impedance networks Z_IN
7