ISL6560 Datasheet, PDF (13/14 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Two-Phase Buck PWM Controller

ISL6560
ISL6560 Supply Design Sequence
Please note several changes from the computations in the
body of the data sheet. An operating frequency of 400kHz
was chosen. A 15mV offset voltage was added to the no-
load output voltage to show the design procedure.
ISL6560 Supply Design Sequence
A. Specifications:
Output Current: 40A
Input Voltage: 12V
Output Voltage: VDAC + 15mV
Output Voltage for Calculations:
V_DAC = 1.8V + 15mV
Droop Voltage: 65mV
Oscillator Frequency: 400kHz (f_SW)
B. Calculate R_OUT:
ROUT
=
V-----_{D----R----O----O-----P--}
I_OUT
=
6----5----m-----V---
40A
=
1.63 m Ω
C. Determine Frequency Setting Capacitor CT:
From curve of Figure A, for 400kHz use 120pF.
3000
1000
100
0
50 100 150 200 250 300 350 400 450 500
CAPACITOR CT (pF)
FIGURE A. OSCILLATOR FREQUENCY vs. TIMING
CAPACITOR
D. Select Inductor Ripple Current (∆I_L):
Choose 40% of IOUT:
∆IL = 40A × 0.4 = 16A
Or 8A / Channel
E. Determine the Inductors:
L
=
V-----I--N------–_{----V----O------U----T-- ×} V-----O-----U-----T--
f_{--S-----W----}
2
×
∆
IL
VIN
=
---1----2----V-----^–-----1---.--8---V------
200kHz × 8A
×
1----.-8----V---
12 V
= 956nH
F. Output Capacitors:
CapacitorESR ≅ ROUT = 1.63mΩ
Sanyo 1500µF, 4V OS-CON Capacitors
have an ESR < 10mΩ
Six capacitors < 1.66mΩ
Total Capacitance = 9mF
G. Input Capacitor’s RMS Current:
Use the curve of Figure B.
0.3
0.2
0.1
0
0
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O / V_IN)
FIGURE B. CURRENT MULTIPLIER vs. DUTY CYCLE
G. Input Capacitor’s RMS Current: (continued)
For 40A with a duty cycle (D) of:
D = ^V-----O----^U----^T-- = ^{1----.--8---V---} = 0.15
V_IN
12V
The multiplier from Figure B is 0.24.
I_RMS = 0.24 × 40A = 9.6A
Panasonic 470µF, 16V Rubycon ZA series capacitors
have a RMS current rating of 1.6A.
Six capacitors were selected.
H. Current Sense Resistor (R_SENSE):
RSENSE
=
-----V----^C----^S----⁽--^T----^H----⁾--^M----^I--^N-------
I--_{O-----U----T- + -I-R----I--P----P----L---E--}
=
---1----4---2----m-----V-----
20A + 8A
=
5.07 m Ω
2
2
Use a 5mΩ resistor
I. R_SENSE Dissipation:
IRMS = I_PEAK D
∴Power = IP2 × D × R_SENSE
Where: I_P = 34.4A -4A = 30.4A
(Using half the ripple current)
Power
=
30.4
2
A
×
0.15
×
5mΩ
Power = 0.69W per channel or 1.38W for both channels
Used two 10mΩ, 1W resistors in parallel.
J. R_L Selection:
RL
=
---n----i---^×-----R----^S----^E----^N----^S----^E-----
gm × R_OUT × 2
=
2----.-2----m----1--S-2---.-×^{-5----1-×--.--6-5--3--m--m---Ω---}Ω^------×^-----2-
=
8.7 k Ω
gm Amplifier Gain = gm × RL = 2.2mS × 8.7k = 19.1
13
FN9011.3