an8041s Panasonic Corporation of North America, an8041s Datasheet - Page 14

• Manufacturer Part Number: an8041s
• Description: Liquid Crystal Backlight Control Ic
• Manufacturer: Panasonic Corporation of North America

AN8041S
14
[3] Function descriptions (continued)
8. Output block, bootstrap circuit (continued)
Application Notes (continued)
2) N-channel MOSFET (M1) turn-on time: t
3) N-channel MOSFET (M1) turn-off time: t
(1) Operating supply voltage range when the step-down circuit is used
(2) Value setting for bootstrap capacitor
(3) CB terminal connection when the booster circuit is used
Bootstrap circuit usage notes
Out terminal voltage V
side voltage after the turn-on rises to the value expressed in the following equation:
terminal voltage is capacitance-coupled, and rises according to the M1 source-side voltage. It is expressed in
the following equation:
coupled, and drops to V
the n-channel MOSFET of the switching device turns on, the voltage of CB terminal (pin 14) rises to the voltage
about two times higher than the V
boost circuit at an operating supply voltage of 3.6 V or more.
to increase the CB terminal voltage over the V
n-channel MOSFET gate drive current. If the capacitance value of the bootstrap capacitor is set at too low
value, it causes the efficiency decrease due to increase in switching loss.
input capacitance.
the n-channel MOSFET source side is grounded. Therefore, use it by short-circuiting the CB terminal (pin
14) to the V
type.
When the PWM comparator output reverses, the Out terminal (pin 13) is switched over to high-level. The
At that time, M1 voltage between the gate and source becomes:
When the Out terminal voltage V
Since the bootstrap capacitor C
The Out terminal voltage drops to the saturation voltage of the transistor Q1 and it is turned off.
The M1 source side voltage decreases to V
When the step-down circuit is used for the DC-DC converter control : As described in the above, when
The bootstrap capacitor is capacitors-coupled with the n-channel MOSFET source-side at its turn-on time
Therefore, set the capacitance at a sufficiently high value compared with the n-channel MOSFET gate
Study with the actual mounting board and set the optimum value.
In the case of using the step-up type DC-DC converter control, the bootstrap circuit is not required since
For that reason, the operating supply voltage range is 3.6 V to 34 V in the case of using the step-up circuit
V
V
V
V
V
V
CB
O
GS
S
CB
CB
CC
V
V
V
V
2 V
2
17 [V]
CC
CC
Ciss
CB
35
O
S
terminal (pin 15).
V
V
V
V
V
CC
CC
CE
F
DS(ON)
CC
V
D1
(sat)
V
V
V
O
D1
CC
D1
D1
rises toward the CB terminal voltage.
V
2
DS(ON)
V
V
V
DS(ON)
V
F
D1
DS(ON)
volt, and returns to the condition described in a).
CC
B
V
O
V
. Since the allowable applied voltage for the CB terminal is 35 V, use the
is connected between the M1 source-side and the CB terminal, the CB
F
rises to the gate threshold voltage, the M1 is turned on. The M1 source-
F
V
[V]
F
2
3
35 [V]
F
CC
, and in the same way the CB terminal voltage is capacitance-
. At this time, the bootstrap capacitor is discharged by the
Voltage Regulators