34704_11 Datasheet, PDF (39/54 Pages) Freescale Semiconductor, Inc – Multiple Channel DC-DC Power Management IC

FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
COMPONENT CALCULATION
F_SW1 AND GENERAL SOFT START
CONFIGURATION
The 34704 uses F_SW1 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 F_SW1, use a 2
resistors voltage divider from VDDI to ground to set the
voltage on the FREQ pin as indicated bellow:
Ratio
F_SW1
[KHz]
Ratio
0
11/32
19/32
VDDI
I_DD max = 100μΑ
Soft Start timing
[ms]
0.5
2.0
8.0
32.0
0
750
9/32
1000
13/32
1250
17/32
1500
21/32
1750
VDDI
2000
Notes
27. If an external voltage is used, F_SW1 can only be set during
device startup.
RF1
V_FREQ
RF2
VDDI
FREQ
V_FREQ = V_{DDI⎝⎛R}-----_F----₁-^R----_+F----²-_R-----_F----₂-_⎠⎞
RF1, RF2 tolerance ±1.0%
GND
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:
RSS1
V_SS
RSS2
VDDI
V_SS
=
V_DDI
⎛
⎝
R^-----S^-----S^--R--1^---S--+^--S---R^--2--S^-----S^----2^{--⎠⎞}
RSS1, RSS2 tolerance ±1.0%
SS
GND
REGULATORS POWER STAGE AND
COMPENSATION CALCULATION
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:
• 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:
RB = -------^R-----¹---------
---^V----^o---- – 1
[Ω]
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:
Analog Integrated Circuit Device Data
Freescale Semiconductor
34704
39