ISL8104 Intersil Corporation, ISL8104 Datasheet - Page 8

• Manufacturer Part Number: ISL8104
• Description: Synchronous Buck Pulse-Width Modulator (PWM) Controller
• Manufacturer: Intersil Corporation

Determine the overcurrent trip point greater than the
maximum output continuous current at maximum inductor
ripple current.
High Speed MOSFET Gate Driver
The integrated driver has the same drive capability and
feature as the Intersil’s 12V gate driver, ISL6612. The PWM
tri-state feature helps prevent a negative transient on the
output voltage when the output is being shut down. This
eliminates the Schottky diode that is used in some systems
for protecting the microprocessor from reversed-output-
voltage damage. See the ISL6612 datasheet for
specification parameters that are not defined in the current
ISL8104 electrical specifications table.
Reference Input
The REFIN pin allows the user to bypass the internal 0.597V
reference with an external reference. If REFIN is NOT above
~2.2V, the external reference pin is used as the control
reference instead of the internal 0.597V reference. When not
using the external reference option the REFIN pin should be
left floating. An internal 6mA pull-up keeps this REFIN pin
above 2.2V in this situation.
Internal Reference and System Accuracy
The Internal Reference is set to 0.597V. The total DC system
accuracy of the system is to be within 1.0% over commercial
temperature range and 1.5% over the industrial temperature
range. System Accuracy includes Error Amplifier offset, and
Reference Error. The use of REFIN may add up to 3mV of
offset error into the system (as the Error Amplifier offset is
trimmed out via the internal System reference.)
Application Guidelines
Layout Considerations
As in any high frequency switching converter, layout is very
important. Switching current from one power device to
another can generate voltage transients across the
impedances of the interconnecting bond wires and circuit
Simple OCP Equation
R
Detailed OCP Equation
R
N
∆I =
F
SW
OCSET
OCSET
U
=
---------------------------------
F
V
=
NUMBER OF HIGH SIDE MOSFETs
SW
IN
Regulator Switching Frequency
=
=
- V
•
L
I
--------------------------------------------------------------- -


--------------------------------------------------------------------------------- -
OC_SOURCE
OUT
I
OUT
OC_SOURCE
•
V
--------------- -
200µA
V
OUT
I
HSOC
IN
r •
+
DS ON
•
I ∆
---- -
2
N


(
U
8
r •
DS ON
)
(
)
ISL8104
traces. These interconnecting impedances should be
minimized by using wide, short printed circuit traces. The
critical components should be located as close together as
possible using ground plane construction or single point
grounding.
A multi-layer printed circuit board is recommended. Figure 5
shows the critical components of the converter. Note that
capacitors C
physical capacitors. Dedicate one solid layer, usually a
middle layer of the PC board, for a ground plane and make
all critical component ground connections with vias to this
layer. Dedicate another solid layer as a power plane and
break this plane into smaller islands of common voltage
levels. Keep the metal runs from the PHASE terminals to the
output inductor short. The power plane should support the
input power and output power nodes. Use copper filled
polygons on the top and bottom circuit layers for the phase
nodes. Use the remaining printed circuit layers for small
signal wiring.
FIGURE 5. PRINTED CIRCUIT BOARD POWER PLANES
KEY
GND
ISL8104
VIA CONNECTION TO GROUND PLANE
TRACE SIZED FOR 3A PEAK CURRENT
SHORT TRACE, MINIMUM IMPEDANCE
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT AND/OR POWER PLANE LAYER
IN
UGATE
PHASE
LGATE
PGND
AND ISLANDS
BOOT
PVCC
and C
VCC
SS
OUT
C
C
C
could each represent numerous
BP_PVCC
BP_VCC
SS
+14V
C
IN
Q
Q
1
2
L
VIN
OUT
C
C
OUT
IN
February 13, 2006
V
OUT
FN9257.0