FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
COMPONENT CALCULATION
F
AND GENERAL SOFT START
SW1
CONFIGURATION
Soft Start timing
[ms]
Ratio
The 34704 uses FSW1 as the switching frequency for
REG1(VG) thru REG5, and this can be changed by applying
a voltage between 0 to 2.5 V to the FREQ pin. If the FREQ
pin is left unconnected, the 34704 starts up with a default
frequency of 750 KHz. To configure the FSW1, use a 2
resistors voltage divider from VDDI to ground to set the
voltage on the FREQ pin as indicated bellow:
0
11/32
0.5
2.0
19/32
8.0
VDDI
32.0
FSW1
Ratio
[KHz]
I
DD max = 100μΑ
0
750
RSS2
RSS1 + RSS2
⎛
⎝
⎞
⎠
---------------------------------------
VSS = VDDI
VDDI
9/32
1000
1250
1500
1750
2000
RSS1
RSS1, RSS2 tolerance ±1.0%
13/32
17/32
21/32
VDDI
V
SS
SS
RSS2
GND
REGULATORS POWER STAGE AND
COMPENSATION CALCULATION
Notes
27. If an external voltage is used, FSW1 can only be set during
device startup.
Regulator 1 and 6 (Synchronous Boost - internally
compensated - REG1 is VG supply).
REG1 is a Synchronous Boost converter set to 5.0 V and
Maximum current of 500 mA while REG6 is set to 15 V at
60 mA (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:
RF2
RF1 + RF2
⎛
⎝
⎞
⎠
-----------------------------
VFREQ = VDDI
VDDI
RF1
RF1, RF2 tolerance ±1.0%
V
FREQ
FREQ
GND
• R1 and RB (Only REG6): These two resistors help to set
the output voltage to the desire value using a Vref=0.6 V,
select R1 between 10 k and 100 K and then calculate RB
as follows:
RF2
Initially at power up, the soft start time will be set for all of
the regulators through programming the SS pin with an
external resistor divider connected between VDDI and AGND
as follows:
R1
---------------------
RB =
[Ω]
Vo
-----------
– 1
Vref
• L: A boost power stage can be designed to operate in
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:
34704
Analog Integrated Circuit Device Data
Freescale Semiconductor
39
FREESCALE [ Freescale ]
