rt9603 ETC-unknow, rt9603 Datasheet - Page 6

• Manufacturer Part Number: rt9603
• Description: Synchronous-rectified Buck Mosfet Drivers
• Manufacturer: ETC-unknow

RT9603
Applications Information
The RT9603 is designed to drive both high side and low
side N-Channel MOSFET through externally input PWM
control signal. It has power-on protection function which
held DRVH and DRVL low before VCC up across the
rising threshold voltage. After the initialization, the PWM
signal takes the control. The rising PWM signal first forces
the DRVL signal turns low then DRVH signal is allowed
to go high just after a non-overlapping time to avoid shoot-
through current. The falling of PWM signal first forces
DRVH to go low. When DRVH and SW signal reach a
predetermined low level, DRVL signal is allowed to turn
high. The non-overlapping function is also presented
between DRVH and DRVL signal transient.
The PWM signal is acted as "High" if above the rising
threshold and acted as "Low" if below the falling threshold.
Any signal level enters and remains within the shutdown
window is considered as "tri-state", the output drivers
are disabled and both MOSFET gates are pulled and
held low. If left the PWM signal (IN) floating, the pin will
be kept at 2.1V by the internal divider and provide the
PWM controller with a recognizable level.
The RT9603 typically operates at frequency of 200kHz
to 250kHz. It shall be noted that to place a 1N4148 or
schottky diode between the VCC and BST pin as shown
in the typical application circuit.
Driving Power MOSFETs
The DC input impedance of the power MOSFET is
extremely high. When V
the current only few nano-amperes. Thus once the gate
has been driven up to "ON" level, the current could be
negligible.
However, the capacitance at the gate to source terminal
should be considered. It requires relatively large currents
to drive the gate up and down 12V (or 5V) rapidly. It also
required to switch drain current on and off with the required
speed. The required gate drive currents are calculated
as follows.
www.richtek.com
6
gs
at 12V (or 5V), the gate draws
Preliminary
Figure 1. Equivalent Circuit and Associated Waveforms
In Figure 1, the current I
the gate up to 12V. The operation consists of charging
C
gate to source of the high side and the low side power
MOSFETs, respectively. In general data sheets, the C
is referred as "C
and C
high side and the low side power MOSFETs, respectively
and referred to the data sheets as "C
transfer capacitance. For example, t
time of the high side and the low side power MOSFETs
respectively, the required current I
below:
l
l
gs1
gs2
gd
V
IN
and C
gd2
C
C
C
gs1
V
V
gs2
are the capacitances from gate to drain of the
gd1
g1
g2
d
gs
1
dVg1
dVg2
. C
V
I
dt
gd1
I
dt
SW
g1
gs1
rss
D
g
+12V
1
1
I
" which is the input capacitance. C
gs1
and C
C
12V
g
C
s
C
2
1
gs1
gs1
gs2
g1
I
gs2
t
g2
r1
t
r2
and I
I
I
gd2
DS9603-00 November 2003
gs2
12
are the capacitances from
12
g2
C
C
gs1
gd2
gs2
are required to move
r1
and I
and t
d
s
2
2
rss
gs2
r2
" the reverse
D
are the rising
L
(2)
,
2
(1)
are showed
t
t
V
GND
OUT
gd1
gs