ISL6527 Intersil Corporation, ISL6527 Datasheet - Page 6

• Manufacturer Part Number: ISL6527
• Description: Single Synchronous Buck Pulse-Width Modulation (PWM) Controller
• Manufacturer: Intersil Corporation

Functional Description
Initialization
The ISL6527 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 when V
is 1.5V
If V
proportionally smaller:
Figure 1 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..
Shoot-Through Protection
A shoot-through condition occurs when both the upper
MOSFET and lower MOSFET are turned on simultaneously,
RiseTime
0V
REF_IN
(1V/DIV)
=
is less that 1.5V, the soft-start rise time will be
t
FIGURE 1. SOFT-START INTERVAL
SS
T0
T1
×
V
-------------------- -
REFIN
1.5V
T2
TIME
6
T3
VOUT (2.50V)
VCC (3.3V)
CPVOUT (5V)
REF_IN
ISL6527
effectively shorting the input voltage to ground. To protect the
regulator from a shoot-through condition, the ISL6527
incorporates specialized circuitry which insures that the
complementary MOSFETs are not ON simultaneously.
The adaptive shoot-through protection utilized by the ISL6527
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 complementary 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
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 2. 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 5kΩ.
R4 can be calculated based on the following equation:
If the output voltage desired is V
back to the FB pin through R1, but do not populate R4.
R4
IN
and the supplied external reference. An external resistor
=
----------------------------------------
V
OUT1
R1
+3.3V
FIGURE 2. OUTPUT VOLTAGE SELECTION
ISL6527
×
VCC
V
–
REF
V
REF
LGATE
CPVOUT
BOOT
UGATE
PHASE
FB
COMP
REF_IN
C2
D1
C4
C1
VIN
R2
REF
Q2
Q1
VREF
LOUT
, simply route the output
COUT
R4
R1
C3
+
R3
VOUT