RT9241A Datasheet, PDF (14/17 Pages) Richtek Technology Corporation – Two-Phase DC/DC Controller for CPU Core Power Supply

RT9241A/B
Design Example for RT9241A
Two phase converter V_CORE = 1.5V, V_IN = 12V, full load
current = 40Amp, droop voltage at full load = 120mV,
OCP trip point for each power stage = 30Amp (at Sample/
Hold), low side MOSFET R_DS(ON) = 6mΩ at room
temperature, L = 2μH, C_OUT = 9000μF, capacitor
ESR = 2mΩ.
1.Compensation setting
a. Modulator Gain, Pole and Zero
VIN
Modulator Gain =
VRAMP
saw-tooth wave amplitude V_RAMP = 1.7V,
modulator Gain = 8.6 = 18.7dB
1
LC filter pole =
= 1.2kHz
2π LC
1
ESR zero = π CRESR = 8.8kHz
2
b. EA compensation network
Use type 2 compensation scheme (see Figure5)
1
FZ =
2π R2C1
1
FP =
2π R2( ^{C1× C2} )
C1+ C2
R2
mid-band gain =
R1
. Choose R₁ = 2.4KΩ,
R₂ = 24KΩ, C₁ = 6.6nF, C₂ = 33pF, get F_Z = 1kHz,
Fp = 200kHz, mid-band Gain = 10 = 20dB, modulator
asymptotic Bode plot of EA compensation and PWM
loop Gain Bode shown as Figure 6.
C₁
R₂
C₂
C₃
R₃
R1
V_CORE
COMP
-
EA₊
R₃, C₃ are used in type 3
compensation scheme
FB ^{(left NC in type 2)}
R_OL
DACOUT
R_OL for no load offset
setting
Figure 5. EA Compensation Network
Asymptotic Bode Plot of PWM Loop Gain
100
80
60
Uncompensated EA Gain
40
20
0
Compensated EA Gain
-20
PWM Loop Gain
-40
Modulator Gain
-60
1₁₀0
1₁0₀₀0
10¹0^K0
10¹0⁰0^K0 10¹0⁰0⁰00^K 10¹0^M0000 10¹0⁰00^M000
Frequency (Hz)
Figure 6. Asymptotic Bode Plot of PWM Loop Gain
2.Over Current Protection setting
OCP trip point current = 30A (at Sample/Hold),
IX = ^{RDS(ON) × 30A} = 75μA ^{, R}ISP ^{= 2.4KΩ}
RSP
Take the temperature rising for consideration, if MOSFET
working temperature = 70°C and the temperature
coefficient = 5000ppm/°C, R_ISP(70°C) = R_ISP(27°C) ×
{R_DS(ON)(70°C)/R_DS(ON)(27°C)} = 1.75KΩ
3.Droop setting
Full load current of each power channel = 40A/2 =
20Amp, the ripple current = ΔI_L =
5μs × ^1.5V × (1- ^1.5V ) = 3.28A
2μH
12V
ΔIL
, load current at S/H 20A - = 18.36A
2
RDS(ON) × 18.36A
= IX(MAX) =
RISP
, GM Amp S/H, R_ISP = R_ISN = 2.4KΩ, I_X(MAX) = 46μA,
required Droop = 120mV = 46μA×2×2/3×R_ADJ,
R_ADJ = 1.97KΩ.
Take the temperature rising for consideration, we just
modify R_ISP like OCP setting.
4.SS capacitor
C_SS = 0.1μF is the suitable value for most application.
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14
DS9241AB-05 March 2007