MAX16930 Datasheet, PDF (22/29 Pages) Maxim Integrated Products – 2MHz, 36V, Dual Buck with Preboost and 20μA Quiescent Current

MAX16930/MAX16931
2MHz, 36V, Dual Buck with Preboost and
20µA Quiescent Current
At the crossover frequency, the total loop gain must be
equal to 1. So:
GAIN_{MOD(fC )}
×
V_FB
V_OUT
× GAIN_{EA(fC )}
= 1
GAIN_E= _A(fC) g_m,EA × R_C
G= AIN_{MOD(fC )}
GAIN_MOD(dc)
×
f_pMOD
f_C
f_SW = 26.4/65.5kI = 0.403MHz
GAIN_MOD(dc) =6.06 × 0.9375 =5.68
= f_pMOD
1
≈ 1.8kHz
2π × 94µF × 0.9375
f_pMOD
<<
f_C
≤
f_SW
5
1.8kHz << f_C ≤ 80.6kHz
Therefore:
select f_C = 40kHz
GAIN_{MOD(fC )}
× ^VFB
V_OUT
× g_m,EA
×R_C
= 1
= f_zMOD
1
≈ 376kHz
2π × 4.5mΩ × 94µF
Solving for R_C:
R_C
=
g_m,EA
V_OUT
× V_FB × GAIN_{MOD(fC )}
since f_zMOD>f_C:
R_C ≈ 16kI
C_C ≈ 5.6nF
C_F ≈ 27pF
Set the error-amplifier compensation zero formed by R_C
and C_C at the f_pMOD. Calculate the value of C_C as follows:
Boost Converter Design Procedure
Setting the Output Voltage in Boost Converter
Adjust the boost converter output voltage by connecting
C_C =
1
2π × fpMOD × RC
a resistive divider from the output of the boost converter
to FBBST to TERM (Figure 3) and R_B2 (FB3 to TERM
resistor). Calculate R_B1 (V_OUT(BOOST) to FBBST resistor)
If f_zMOD is less than 5 x f_C, add a second capacitor C_F
using the following equation:
from COMP to AGND. The value of C_F is:
C_F =
1
= R_B1
R
B2


V_{OUT (BOOST}
V_FB3
)


−

1

2π × fzMOD × RC
where V_FB3 = 1.2V (typ) (see the Electrical Characteristics
table).
As the load current decreases, the modulator pole also
decreases; however, the modulator gain increases accord-
ingly and the crossover frequency remains the same.
Below is a numerical example to calculate the compen-
VOUT(BOOST)
sation network component values of Figure 2:
A_{V_CS} = 11V/V
MAX16930/
RB1
MAX16931
R_DCR = 15mI
FB3
g_mc = 1/(A_{V_CS} x R_DC) = 1/(11 x 0.015) = 6.06
V_OUT = 5V
RB2
I_OUT(MAX) = 5.33A
TERM
R_LOAD = V_OUT/I_OUT(MAX) = 5V/5.33A = 0.9375I
C_OUT = 2x47µF = 94µF
ESR = 9mI/2 = 4.5mI
Figure 3. Boost Converter Adjustable Output Voltage
Maxim Integrated
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