ISL6526 Datasheet, PDF (13/15 Pages) Intersil Corporation – Single Synchronous Buck Pulse-Width Modulation PWM Controller

ISL6526, ISL6526A
where Q_GATE is the maximum total gate charge of the upper
MOSFET, C_BOOT is the bootstrap capacitance, V_BOOT1 is
the bootstrap voltage immediately before turn-on, and
V_BOOT2 is the bootstrap voltage immediately after turn-on.
The bootstrap capacitor begins its refresh cycle when the gate
drive begins to turn-off the upper MOSFET. A refresh cycle
ends when the upper MOSFET is turned on again, which
varies depending on the switching frequency and duty cycle.
The minimum bootstrap capacitance can be calculated by
rearranging the previous equation and solving for C_BOOT.
C_BOOT
=
-----------------Q-----^G----^A----^T----^E------------------
V_B
O
O
T
1
–
V
B
OOT2
(EQ. 19)
Typical gate charge values for MOSFETs considered in
these types of applications range from 20 to 100nC. Since
the voltage drop across Q_LOWER is negligible, V_BOOT1 is
simply V_CPVOUT - V_D. A Schottky diode is recommended to
minimize the voltage drop across the bootstrap capacitor
during the on-time of the upper MOSFET. Initial calculations
with V_BOOT2 no less than 4V will quickly help narrow the
bootstrap capacitor range.
For example, consider an upper MOSFET is chosen with a
maximum gate charge, Q_g, of 100nC. Limiting the voltage
drop across the bootstrap capacitor to 1V results in a value
of no less than 0.1µF. The tolerance of the ceramic capacitor
should also be considered when selecting the final bootstrap
capacitance value.
A fast recovery diode is recommended when selecting a
bootstrap diode to reduce the impact of reverse recovery
charge loss. Otherwise, the recovery charge, Q_RR, would
have to be added to the gate charge of the MOSFET and
taken into consideration when calculating the minimum
bootstrap capacitance.
ISL6526, ISL6526A DC/DC Converter
Application Circuit
Figure 8 shows an application circuit of a DC/DC Converter.
Detailed information on the circuit, including a complete Bill
of Materials and circuit board description, can be found in
Application Note AN1021:
http://www.intersil.com/data/an/an1021.pdf.
3.3V
GND
TP₁
4
CT1
C₁
0.1µF
11
VCC
U₁
OCSET ⁶
C₄
0.22µF
ISL6526, ISL6526A
5
CT2
3
CPVOUT
10
CPGND
BOOT ¹³
1
GND
14
UGATE
12
PHASE
ENABLE
9
ENABLE
LGATE ²
COMP
FB
8
C₁₀
7
33pF
R₂
C₁₁
6.49kΩ 5600pF
R₅
1.07kΩ
C₂
1000pF
TP₃
R₁
9.76kΩ
C₅
D₁
10µF
CERAMIC
CAP
C_{7 0.1µF}
C₃
C₆
1µF
L₁
C_{8, 9}
Q₁
R₃
2.26kΩ
R₄ C₁₂
124Ω 8200pF
2.5V @ 5A
GND
Component Selection Notes:
FIGURE 8. 3.3V TO 2.5V 5A DC/DC CONVERTER
C_3, C_8, C₉ - Each 150µF, Panasonic EEF-UE0J151R or Equivalent.
D1 - 30mA Schottky Diode, MA732 or Equivalent
L₁ - 1µH Inductor, Panasonic P/N ETQ-P6F1ROSFA or Equivalent.
Q₁- Fairchild MOSFET; ITF86110DK8.
13
FN9055.10
November 24, 2008