ISL6554 Datasheet, PDF (14/16 Pages) Intersil Corporation – Microprocessor CORE Voltage Regulator Using Multi-Phase Buck PWM Control Without Programmable Droop

ISL6554
+5V_IN
C_BP
C_T
R_FB
LOCATE NEXT
TO FB PIN
R_IN
C_BP
LOCATE NEXT TO IC PIN(S)
+12V
VCC PVCC
VCC
PWM
HIP6601
COMP FS/DIS
ISL6554
R_T
FB
VSEN
LOCATE NEXT TO IC PIN
R_SEN
ISEN
USE INDIVIDUAL METAL RUNS
FOR EACH CHANNEL TO HELP
ISOLATE OUTPUT STAGES
C_BOOT
PHASE
C_IN
L_O1
LOCATE NEAR TRANSISTOR
V_CORE
C_OUT
KEY
ISLAND ON POWER PLANE LAYER
ISLAND ON CIRCUIT PLANE LAYER
VIA CONNECTION TO GROUND PLANE
FIGURE 12. PRINTED CIRCUIT BOARD POWER PLANES AND ISLANDS
Input Capacitor Selection
The important parameters for the bulk input capacitors are the
voltage rating and the RMS current rating. For reliable
operation, select bulk input capacitors with voltage and current
ratings above the maximum input voltage and largest RMS
current required by the circuit. The capacitor voltage rating
should be at least 1.25 times greater than the maximum input
voltage and a voltage rating of 1.5 times is a conservative
guideline. The RMS current required for a multi-phase
converter can be approximated with the aid of Figure 13.
0.5
SINGLE
0.4
CHANNEL
0.3
2 CHANNEL
0.2
3 CHANNEL
0.1
4 CHANNEL
0
0
0.1
0.2
0.3
0.4
0.5
DUTY CYCLE (V_O/V_IN)
FIGURE 13. CURRENT MULTIPLIER vs DUTY CYCLE
First determine the operating duty ratio as the ratio of the
output voltage divided by the input voltage. Find the Current
Multiplier from the curve with the appropriate power
channels. Multiply the current multiplier by the full load
output current. The resulting value is the RMS current rating
required by the input capacitor.
Use a mix of input bypass capacitors to control the voltage
overshoot across the MOSFETs. Use ceramic capacitance for
the high frequency decoupling and bulk capacitors to supply
the RMS current. Small ceramic capacitors should be placed
very close to the drain of the upper MOSFET to suppress the
voltage induced in the parasitic circuit impedances.
For bulk capacitance, several electrolytic capacitors (Panasonic
HFQ series or Nichicon PL series or Sanyo MV-GX or
equivalent) may be needed. For surface mount designs, solid
tantalum capacitors can be used, but caution must be
exercised with regard to the capacitor surge current rating.
These capacitors must be capable of handling the surge-
current at power-up. The TPS series available from AVX, and
the 593D series from Sprague are both surge current tested.
MOSFET Selection and Considerations
In high-current PWM applications, the MOSFET power
dissipation, package selection and heatsink are the
dominant design factors. The power dissipation includes two
loss components; conduction loss and switching loss. These
losses are distributed between the upper and lower
MOSFETs according to duty factor (see the following
equations). The conduction losses are the main component
of power dissipation for the lower MOSFETs, Q2 and Q4 of
Figure 1. Only the upper MOSFETs, Q1 and Q3 have
significant switching losses, since the lower device turns on
and off into near zero voltage.
The equations assume linear voltage-current transitions and
do not model power loss due to the reverse-recovery of the
lower MOSFETs body diode. The gate-charge losses are
dissipated by the Driver IC and don’t heat the MOSFETs.
However, large gate-charge increases the switching time,
14
FN9003.3
February 11, 2005