34704_11 Datasheet, PDF (41/54 Pages) Freescale Semiconductor, Inc – Multiple Channel DC-DC Power Management IC

1. First calculate the dt_I (inductor current rising time)
given by:
dtI = --^I--^o----^m----^a---^x---^T---
[s]
ΔIostep
Where the parameter ΔIo_step is the maximum current
step during the current rising time and is define as:
ΔIostep
=
⎛
⎝
-^D-_F---^m-_s---_w^a---^x-_⎠⎞
⎛
⎝
V_-----i_--n_{---m----i-L-n----}–_-----V_----o_{--⎠⎞}
[A]
2. Then the output capacitor can be chosen as follow:
C_OUT
≤
I_---o_{---m----a---x---}d_----t_--I
ΔVo_max
[A]
• Where ΔVO_max is the maximum allowed transient
overshoot expressed as a percentage of the output
voltage, typically from 3 to 5% of Vo.
3. Finally find the maximum allowed ESR to allow the
desired transient response:
ESR_max
=
Δ_-----V_----o_---r--(_---F_----s_---w_-----)_--(_---L_---)_-
Vo(1 – D_min)
[Ω]
NOTE: Do not use the parameters ΔVO_r and ΔVO_max
indistinctly, the first one indicates the output voltage ripple,
while the second one is the maximum output voltage
overshoot (transient response).
FUNCTIONAL DEVICE OPERATION
COMPONENT CALCULATION
• R1 and RB: 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
• Compensation network. (C1,C2,C3, R2, R3): For
compensating a buck converter, 3 important frequencies
referring to the plant are:
1. Output LC filter cutoff frequency (F_LC):
F_LC = ------------¹--------------
[Hz]
2π _LCOUT
2. Cutoff frequency due to capacitor ESR:
F_ESR = -₂---_π----₍--_C-----_O----_U¹----_T---₎--_E-----_S----_R---
[Hz]
3. Crossover frequency (or bandwidth):
F_BW
=
-^F---^S----^W---
10
[Hz]
The Type 3 external compensation network will be in
charge of canceling some of these poles and zeros to
achieve stability in the system. The following poles and
zeroes frequencies are provided by the type 3 compensation.
Analog Integrated Circuit Device Data
Freescale Semiconductor
34704
41