ISL8104EVAL1Z Intersil, ISL8104EVAL1Z Datasheet - Page 12

• Manufacturer Part Number: ISL8104EVAL1Z
• Description: EVALUATION BOARD FOR ISL8104
• Manufacturer: Intersil

Specifications of ISL8104EVAL1Z

• Main Purpose: DC/DC, Step Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 1.8V
• Current - Output: 20A
• Voltage - Input: 8 ~ 14.4V
• Regulator Topology: Buck
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: ISL8104
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Power - Output: -

Standard-gate MOSFETs are normally recommended for
use with the ISL8104. However, logic-level gate MOSFETs
can be used under special circumstances. The input voltage,
top-side gate drive level, and the MOSFETs absolute gate-
to-source voltage rating determine whether logic-level
MOSFETs are appropriate.
Figure 9 shows the top-side gate drive (BOOT pin) supplied
by a bootstrap circuit from +14V. The boot capacitor, C
develops a floating supply voltage referenced to the LX pin.
This supply is refreshed each cycle to a voltage of +14V less
the boot diode drop (V
turns on. A MOSFET can only be used for Q
MOSFETs absolute gate-to-source voltage rating exceeds
the maximum voltage applied to +14V. For Q
MOSFET can be used if its absolute gate-to-source voltage
rating also exceeds the maximum voltage applied to +14V.
Figure 10 shows the top-side gate drive supplied by a direct
connection to +14V. This option should only be used in
converter systems where the main input voltage is +5VDC or
less. The peak top-side gate-to-source voltage is
approximately +14V less the input supply. For +5V main
power and +14VDC for the bias, the gate-to-source voltage
of Q
and a logic-level MOSFET can be used for Q
gate-to-source voltage rating exceeds the maximum voltage
applied to PVCC. This method reduces the number of
required external components, but does not provide for
immunity to phase node ringing during turn on and may
result in lower system efficiency.
Schottky Selection
Rectifier D2 is a clamp that catches the negative inductor
swing during the dead time between turning off the bottomside
MOSFET and turning on the top-side MOSFET. The diode
must be a Schottky type to prevent the lossy parasitic
MOSFET body diode from conducting. It is acceptable to omit
the diode and let the body diode of the bottom-side MOSFET
clamp the negative inductor swing, but efficiency could slightly
decrease as a result. The diode's rated reverse breakdown
voltage must be greater than the maximum input voltage.
1
is 9V. A logic-level MOSFET is a good choice for Q
D
) when the bottom-side MOSFET, Q
12
1
2
2
, a logic-level
if the
if its absolute
BOOT
1
2
ISL8104
FIGURE 10. TOP-SIDE GATE DRIVE - DIRECT V
FIGURE 9. TOP-SIDE GATE DRIVE - BOOTSTRAP OPTION
+
+
-
-
ISL8104
ISL8104
+14V
+14V
OPTION
+
GND
D
GND
V
BOOT
D
BOOT
TGATE
BGATE
PGND
BOOT
TGATE
BGATE
PGND
PVCC
PVCC
LX
-
+14V
+14V
C
BOOT
Q1
Q2
Q1
Q2
+1.2V TO +14V
+5V OR LESS
D2
D2
V
NOTE:
V
NOTE:
NOTE:
V
NOTE:
V
G-S
G-S
G-S
G-S
CC
≈
≈
≈
≈
DRIVE
March 7, 2008
V
V
PVCC
PVCC
CC
CC
FN9257.2
- 5V
- V
D