ISL6406_07 Datasheet, PDF (8/18 Pages) Intersil Corporation – Single Synchronous Buck Pulse-Width Modulation (PWM) Controller

ISL6406
Functional Description
Initialization
The ISL6406 automatically initializes upon receipt of power.
Special sequencing of the input supplies is not necessary.
The Power-On Reset (POR) function continually monitors the
the output voltage of the charge pump. During POR, the charge
pump operates on a free running oscillator. Once the POR level
is reached, the charge pump oscillator is synched to the PWM
oscillator. The POR function also initiates the soft-start
operation after the charge pump output voltage exceeds its
POR threshold.
Soft-Start
The POR function initiates the digital soft-start sequence.
The PWM error amplifier reference is clamped to a level
proportional to the soft-start voltage. As the soft-start voltage
slews up, the PWM comparator generates PHASE pulses of
increasing width that charge the output capacitor(s). This
method provides a rapid and controlled output voltage rise.
The soft start sequence typically takes about 6.5ms.
(1V/DIV)
CPVOUT (5V)
VCC (3.3V)
V_OUT (2.50V)
0V
t0 t2
t1
t3
TIME
FIGURE 2. SOFT-START INTERVAL
Figure 2 shows the soft-start sequence for a typical
application. At t0, the +3.3V VCC voltage starts to ramp. At
time t1, the Charge Pump begins operation and the +5V
CPVOUT IC bias voltage starts to ramp up. Once the voltage
on CPVOUT crosses the POR threshold at time t2, the
output begins the soft-start sequence. The triangle waveform
from the PWM oscillator is compared to the rising error
amplifier output voltage. As the error amplifier voltage
increases, the pulse-width on the UGATE pin increases to
reach the steady-state duty cycle at time t3.
Frequency Selection
The ISL6406 offers adjustable frequency from 100kHz to
700kHz by changing external resistor connected at pin RT.
Figure 3 shows the typical RT vs Frequency variation curve.
200
180
160
140
120
100
80
60
40
20
50 100 150 200 250 300 350 400 450 500 550 600 650 700 750
FREQUENCY (kHz)
FIGURE 3. RT vs FREQUENCY
Shoot-Through Protection
A shoot-through condition occurs when both the upper
MOSFET and lower MOSFET are turned on simultaneously,
effectively shorting the input voltage to ground. To protect
the regulator from a shoot-through condition, the ISL6406
incorporates specialized circuitry which insures that the
MOSFETs are not ON simultaneously.
The adaptive shoot-through protection utilized by the
ISL6406 looks at the lower gate drive pin, LGATE, and the
upper gate drive pin, UGATE, to determine whether a
MOSFET is ON or OFF. If the voltage from UGATE or from
LGATE to GND is less than 0.8V, then the respective
MOSFET is defined as being OFF and the other MOSFET is
turned ON. This method of shoot-through protection allows
the regulator to sink or source current.
Since the voltage of the lower MOSFET gate and the upper
MOSFET gate are being measured to determine the state of
the MOSFET, the designer is encouraged to consider the
repercussions of introducing external components between
the gate drivers and their respective MOSFET gates before
actually implementing such measures. Doing so may
interfere with the shoot-through protection.
Output Voltage Selection
The output voltage can be programmed to any level between
V_IN and the internal reference, 0.8V. An external resistor
divider is used to scale the output voltage relative to the
reference voltage and feed it back to the inverting input of
the error amplifier, see Figure 4. However, since the value of
R1 affects the values of the rest of the compensation
components, it is advisable to keep its value less than 5K.
R4 can be calculated based on Equation 2:
R4 = _V-----O--⁽--R_U----_T-1---₁-⁾--⁽-_–-0----.₍-8-0---V-.-8---⁾-V-----₎
(EQ. 2)
If the output voltage desired is 0.8V, simply route the output
back to the FB pin through R1, but do not populate R4.
8
FN9073.7
January 16, 2007