參數(shù)資料
型號(hào)： ISL6522IRZ-T
廠商： INTERSIL CORP
元件分類： 穩(wěn)壓器
英文描述： Buck and Synchronous Rectifier Pulse-Width Modulator (PWM) Controller
中文描述： SWITCHING CONTROLLER, 1000 kHz SWITCHING FREQ-MAX, PQCC16
封裝： 5 X 5 MM, ROHS COMPLIANT, PLASTIC, MO-220-VHHB, QFN-16
文件頁(yè)數(shù)： 10/14頁(yè)
文件大小： 473K
代理商： ISL6522IRZ-T
10
FN9030.7
March 4, 2005
switch realizes most of the switching losses when the converter
is sinking current (see the equations below).
These equations assume linear voltage-current transitions
and do not adequately model power loss due the reverse-
recovery of the upper and lower MOSFET’s body diode. The
gate-charge losses are dissipated by the ISL6522 and do not
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.
Standard-gate MOSFETs are normally recommended for
use with the ISL6522. However, logic-level gate MOSFETs
can be used under special circumstances. The input voltage,
upper gate drive level, and the MOSFETs absolute gate-to-
source voltage rating determine whether logic-level
MOSFETs are appropriate.
Figure 9 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. This supply is refreshed each cycle to a voltage of V
CC
less the boot diode drop (V
D
) when the lower MOSFET, Q2
turns on. A logic-level MOSFET can only be used for Q1 if
the MOSFETs absolute gate-to-source voltage rating
exceeds the maximum voltage applied to V
CC
. For Q2, a
logic-level MOSFET can be used if its absolute gate-to-
source voltage rating exceeds the maximum voltage applied
to PVCC.
Figure 10 shows the upper gate drive supplied by a direct
connection to V
CC
. This option should only be used in
converter systems where the main input voltage is +5V
DC
or
less. The peak upper gate-to-source voltage is approximately
V
CC
less the input supply. For +5V main power and +12V
DC
for the bias, the gate-to-source voltage of Q1 is 7V. A logic-level
MOSFET is a good choice for Q1 and a logic-level MOSFET
can be used for Q2 if its absolute gate-to-source voltage rating
exceeds the maximum voltage applied to PV
CC
.
Schottky Selection
Rectifier D2 is a clamp that catches the negative inductor
swing during the dead time between turning off the lower
MOSFET and turning on the upper 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 lower MOSFET clamp the
negative inductor swing, but efficiency will drop one or two
percent as a result. The diode's rated reverse breakdown
voltage must be greater than the maximum input voltage.
P
LOWER
= Io
2
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.
Losses while Sourcing Current
Io
2
=
Losses while Sinking Current
P
UPPER
= Io
2
x r
DS(ON)
x D
P
LOWER
Io
2
r
DS ON
(
)
×
1
D
–
(
)
×
1
V
IN
×
t
SW
F
S
×
×
+
=
P
UPPER
r
DS ON
)
×
D
×
1
V
IN
×
t
SW
F
S
×
×
+
+12V
PGND
ISL6522
GND
LGATE
UGATE
PHASE
BOOT
VCC
+5V OR +12V
FIGURE 9. UPPER GATE DRIVE - BOOTSTRAP OPTION
NOTE:
V
G-S
≈
V
CC
- V
D
NOTE:
V
G-S
≈
PVCC
C
BOOT
D
BOOT
Q1
Q2
PVCC
+5V
OR +12V
D2
+
-
V
D
+
-
+12V
PGND
LGATE
UGATE
PHASE
BOOT
VCC
+5V OR LESS
FIGURE 10. UPPER GATE DRIVE - DIRECT V
CC
DRIVE OPTION
NOTE:
V
G-S
≈
V
CC
- 5V
NOTE:
V
G-S
≈
PVCC
Q1
Q2
PVCC
+5V
OR +12V
D2
ISL6522
GND
+
-
ISL6522
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