ISL6520 Datasheet, PDF (8/11 Pages) Intersil Corporation – Single Synchronous Buck Pulse-Width Modulation (PWM) Controller

ISL6520
minimum and maximum output levels for the worst case
response time.
Input Capacitor Selection
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use small ceramic
capacitors for high frequency decoupling and bulk
capacitors to supply the current needed each time Q₁ turns
on. Place the small ceramic capacitors physically close to
the MOSFETs and between the drain of Q₁ and the source
of Q₂.
The important parameters for the bulk input capacitor are the
voltage rating and the RMS current rating. For reliable
operation, select the bulk capacitor with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25 times greater than the maximum
input voltage and a voltage rating of 1.5 times is a
conservative guideline. The RMS current rating requirement
for the input capacitor of a buck regulator is approximately
1/2 the DC load current.
For a through hole design, several electrolytic capacitors
may be needed. For surface mount designs, solid tantalum
capacitors can be used, but caution must be exercised with
regard to the capacitor surge current rating. These
capacitors must be capable of handling the surge-current at
power-up. Some capacitor series available from reputable
manufacturers are surge current tested.
MOSFET Selection/Considerations
The ISL6520 requires two N-Channel power MOSFETs.
These should be selected based upon r_DS(ON), gate
supply requirements, and thermal management
requirements.
In high-current applications, the MOSFET power
dissipation, package selection and heatsink are the
dominant design factors. The power dissipation includes
two loss components; conduction loss and switching loss.
The conduction losses are the largest component of power
dissipation for both the upper and the lower MOSFETs.
These losses are distributed between the two MOSFETs
according to duty factor (see the equations below). Only
the upper MOSFET has switching losses, since the lower
MOSFETs body diode or an external Schottky rectifier
across the lower MOSFET clamps the switching node
before the synchronous rectifier turns on. These equations
assume linear voltage-current transitions and do not
adequately model power loss due the reverse-recovery of
the lower MOSFET’s body diode. The gate-charge losses
are dissipated by the ISL6520 and don't heat the
MOSFETs. However, large gate-charge increases the
switching interval, t_SW which increases the upper MOSFET
switching losses. Ensure that both MOSFETs are within
their maximum junction temperature at high ambient
temperature by calculating the temperature rise according
to package thermal-resistance specifications. A separate
heatsink may be necessary depending upon MOSFET
power, package type, ambient temperature and air flow.
P_UPPER = Io² x r_DS(ON) x D +
1
2
Io x V_IN x t_SW x F_S
P_LOWER = Io² x r_DS(ON) x (1 - D)
Where: D is the duty cycle = V_OUT / V_IN,
t_SW is the switching interval, and
F_S is the switching frequency.
Given the reduced available gate bias voltage (5V),
logic-level or sub-logic-level transistors should be used for
both N-MOSFETs. Caution should be exercised with
devices exhibiting very low V_GS(ON) characteristics. The
shoot-through protection present aboard the ISL6520 may
be circumvented by these MOSFETs if they have large
parasitic impedences and/or capacitances that would
inhibit the gate of the MOSFET from being discharged
below its threshold level before the complementary
MOSFET is turned on.
+5V
D_BOOT
VCC
ISL6520
+ V_D -
+5V
BOOT
C_BOOT
Q1
UGATE
PHASE
-
Q2
LGATE
+
GND
NOTE:
V_G-S ≈ V_CC -V_D
NOTE:
V_G-S ≈ V_CC
FIGURE 7. UPPER GATE DRIVE BOOTSTRAP
Figure 7 shows the upper gate drive (BOOT pin) supplied
by a bootstrap circuit from V_CC. The boot capacitor,
C_BOOT, develops a floating supply voltage referenced to
the PHASE pin. The supply is refreshed to a voltage of V_CC
less the boot diode drop (V_D) each time the lower
MOSFET, Q₂, turns on.
8
FN9009.4
October 4, 2005