MC34717EP Freescale Semiconductor, MC34717EP Datasheet - Page 21

• Manufacturer Part Number: MC34717EP
• Description: IC CONVERTER DDR 26QFN
• Manufacturer: Freescale Semiconductor

Specifications of MC34717EP

• Applications: Converter, DDR
• Voltage - Input: 3 ~ 6 V
• Number Of Outputs: 2
• Voltage - Output: 0.7 ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 26-QFN
• Output Voltage: 0.7 V to 3.6 V
• Output Current: 5 A
• Input Voltage: 3 V to 6 V
• Switching Frequency: 1 MHz
• Operating Temperature Range: - 40 C to + 85 C
• Mounting Style: SMD/SMT
• No. Of Outputs: 2
• Power Dissipation Pd: 2.03W
• No. Of Pins: 26
• Filter Terminals: SMD
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

SELECTING INDUCTOR
Channels. The equation is the following:
L
SELECTING THE OUTPUT FILTER CAPACITOR
output capacitor, and not the actual Farad value: the physical
size, the ESR of the capacitor, and the voltage rating.
following formula:
transient response.)
Analog Integrated Circuit Device Data
Freescale Semiconductor
D
I Δ
Rds
T
r _
=
The Inductor calculation process is the same for both
The following considerations are most important for the
Calculate the minimum output capacitor using the
Transient Response percentage:
TR_%
(Use a recommended value of 2 to 4% to assure a good
Maximum Transient Voltage:
TR_V_dip = V
Maximum Current Step:
Inductor Current Rise Time:
'
MAX
OUT
Δ
D
w
(
Iout
on
'
MAX
=
_ )
1
_
−
∗
Co
T
ls
step
Vin
∗
=
OUT
V
(
V
_
OUT
I
=
OUT
OUT
max
*TR_%
(
TR
Vin
+
*
_
dt
I
_
OUT
V
Maximum Off Time Percentage
Switching Period
Drain – to – Source
Resistance of FET
Winding Resistance of Inductor
Output Current Ripple
min
_
_
I
( *
dip
_
−
Fsw
Rds
Δ
rise
Vout
I
OUT
*
(
on
L
* )
_ )
D
ls
_
+
max
r
_
w
))
allowed ESR.
ers and may present a hidden danger to the ultimate supply
stability. Poor quality capacitors have a widely disparate
ESR value, which can make the closed loop response incon-
sistent.
BOOTSTRAP CAPACITOR
voltage for the high side MOSFET. This N-Channel MOSFET
needs a voltage difference between its gate and source to be
able to turn on. The high side MOSFET source is the SW
node, so it is not at ground and it is floating and shifting in
voltage. We cannot just apply a voltage directly to the gate of
the high side that is referenced to ground. We need a voltage
referenced to the SW node. This is why the bootstrap
capacitor is needed. This capacitor charges during the high
side off time. The low side will be on during that time. The SW
node and the bottom of the bootstrap capacitor will be
connected to ground, and the top of the capacitor will be
connected to a voltage source. The capacitor will charge up
to that voltage source (for example 5.0 V). Now when the low
side MOSFET switches off and the high side MOSFET
switches on, the SW nodes will rise to V
the boot pin will be V
have V
0.1 μF capacitor is a good value for this bootstrap element.
TYPE III COMPENSATION NETWORK
regulation output voltages. A high DC gain is required to
accomplish this, but with high gain comes the possibility of
instability. The purpose of adding compensation to the
internal error amplifier is to counteract some of the gains and
phases contained in the control-to-output transfer function
that could jeopardized the stability of the power supply. The
Type III compensation network used for the 34717 comprises
two poles (one integrator and one high frequency to cancel
the zero generated from the ESR of the output capacitor) and
two zeros to cancel the two poles generated from the LC filter
as shown in
The following formula is helpful to find the maximum
The bootstrap capacitor is needed to supply the gate
Power supplies are desired to offer accurate and tight
The effects of the ESR is often neglected by the design-
CAP
across it and it will be able to stay enhanced. A
ESR
Figure
dt
_
max
I
9.
CAP
_
=
rise
Δ
V
+ V
OUT
V
OUT
=
IN
1 (
. The gate of the high side will
Δ
*
−
I
T
OUT
Fsw
D
*
TYPICAL APPLICATIONS
min)
I
_
IN
OUT
*
, and the voltage on
step
L
34717
21