
Using
’
P
’
Channel Devices (No Bootstrap)
All the preceeding examples utilise N channel
MOSFET devices and a bootstrap circuit to provide full
enhancement to the high side device. These circuits
have a maximum input voltage of 10V. For
applications requiring a higher input voltage, using P
channel devices for the main MOSFET will allow up to
18V operation. Typically this may be in a 12V to 5V
converter circuit.
If the same package size MOSFET devices are used, it
is likely a higher on resistance will be encountered,
with the result that efficiency will decline slightly.
Fig 9 shows the efficiency plot for a P phase
synchronous 5V converter based on the
ZXRD1050PQ16. The figure charts efficiency v output
current at 12V input and 7V input.
50
55
60
65
70
75
80
85
90
95
100
0.1
1
10
Efficiency v I
V
OUT
=3.3V.
OUT
I
(A)
OUT
V
=5V
IN
E
Fig.9
’
P
’
Channel Device Efficiency (synchronous)
50
55
60
65
70
75
80
85
90
95
100
0.1
1
10
Efficiency v I
V
OUT
=5V.
OUT
I
(A)
OUT
V
=7V
IN
V
=12V
IN
E
23
Fig.8 Efficiency for non-synchronous 5-3.3V conversion
ZXRD1000 SERIES
ISSUE 4 - OCTOBER 2000