MC34704 FREESCALE [Freescale Semiconductor, Inc], MC34704 Datasheet - Page 36

• Manufacturer Part Number: MC34704
• Description: Multiple Channel DC-DC Power Management IC
• Manufacturer: FREESCALE [Freescale Semiconductor, Inc]

F
CONFIGURATION
REG1(VG) thru REG5, and this can be changed by applying
a voltage between 0 to 2.5V to the FREQ pin. If the FREQ pin
is left unconnected, the 34704 starts up with a default
frequency of 750KHz. To configure the F
resistors voltage divider from VDDI to ground to set the
voltage on the FREQ pin as indicated bellow:
the regulators through programming the SS pin with an
external resistor divider connected between VDDI and AGND
as follows:
36
34704
FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
SW1
1.
The 34704 uses F
Initially at power up, the soft start time will be set for all of
• Now calculate the maximum allowed ESR to reach the
V
I
FREQ
DD
RF2
AND GENERAL SOFT START
RF1
If an external voltage is used, F
device startup.
max = 100
Ratio
13/32
17/32
21/32
Ratio
11/32
19/32
VDDI
VDDI
9/32
0
0
VDDI
FREQ
GND
SW1
as the switching frequency for
V
FREQ
RF1, RF2 tolerance
SW1
Soft Start timing
=
can only be set during
V
SW1
[KHz]
DDI
F
1000
1250
1500
1750
2000
[ms]
32.0
750
0.5
2.0
8.0
SW1
, use a 2
⎛
⎝
COMPONENT CALCULATION
--------------------------- -
RF1
RF2
+
RF2
⎞
⎠
REGULATORS POWER STAGE AND
COMPENSATION CALCULATION
Regulator 1 and 6 (Synchronous Boost - internally
compensated - REG1 is VG supply).
Maximum current of 500mA while REG6 is set to 15V at
60ma(on the 34704B, REG1 does not exist but similar
circuitry is used to provide the internal VG voltage). They do
not need an external compensation network, thus, the only
components that need to be calculated are:
• L : A boost power stage can be designed to operate in
inductor rated between 50 to 100mA in order to have this
regulator working in DCM. Rising the inductor value will make
the regulator to begin working in CCM.
• C
L
C
L
1. Define I
2. However the worst case condition for the boost power
REG1 is a Synchronous Boost converter set to 5V and
CCM for load currents above a certain level usually 5 to
15% of full load. The minimum value of inductor to
maintain CCM can be determined by using the following
procedure:
Note: On the 34704B Use the recommended 3.0uH
contribute to its impedance and output voltage ripple are
the ESR, the ESL and the capacitance C. The minimum
capacitor value is approximately:
• Where
min
OUT
min
OUT
RSS1
RSS2
15% of full load.
stage is when the input voltage is equal to one half of
the output voltage, which results in the Maximum
then:
V
SS
: The three elements of output capacitor that
Vo D 1 D
----------------------------------------- -
Vo T
--------------- -
16I
Io
--------------------------- -
Fsw Vo
max
OB
OB
VO
2I
D
as the minimum current to maintain CCM as
OB
max
r
–
VDDI
SS
GND
r
is the desired output voltage ripple.
Analog Integrated Circuit Device Data
2
T
V
RSS1, RSS2 tolerance
SS
=
Freescale Semiconductor
V
DDI
⎛
⎝
-----------------------------------
RSS1
RSS2
+
(H)
(H)
RSS2
(F)
⎞
⎠
I
L
,