ISL95870HRUZ-T Intersil, ISL95870HRUZ-T Datasheet - Page 23

• Manufacturer Part Number: ISL95870HRUZ-T
• Description: IC CTRLR PWM 1PHASE GPU 16UTQFN
• Manufacturer: Intersil

Specifications of ISL95870HRUZ-T

• Applications: Controller, GPU Core Power
• Voltage - Input: 3.3 V ~ 25 V
• Number Of Outputs: 1
• Voltage - Output: 0.5 V ~ 5 V
• Operating Temperature: -10°C ~ 100°C
• Mounting Type: *
• Package / Case: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

A DC/DC buck regulator must have output capacitance
C
develops a corresponding ripple voltage V
which is the sum of the voltage drop across the capacitor
ESR and of the voltage change stemming from charge
moved in and out of the capacitor. These two voltages
are expressed in Equations 39 and 40:
ΔV
If the output of the converter has to support a load with
high pulsating current, several capacitors will need to be
paralleled to reduce the total ESR until the required V
is achieved. The inductance of the capacitor can
significantly impact the output voltage ripple and cause
a brief voltage spike if the load transient has an
extremely high slew rate. Low inductance capacitors
should be considered. A capacitor dissipates heat as a
function of RMS current and frequency. Be sure that I
is shared by a sufficient quantity of paralleled capacitors
so that they operate below the maximum rated RMS
current at F
a capacitor can fade as much as 50% as the DC voltage
across it increases.
Selecting the Input Capacitor
The important parameters for the bulk input capacitors
are the voltage rating and the RMS current rating. For
reliable operation, select bulk capacitors with voltage and
current ratings above the maximum input voltage and
capable of supplying the RMS current required by the
switching circuit. Their voltage rating should be at least
1.25x greater than the maximum input voltage, while a
voltage rating of 1.5x is a preferred rating. Figure 19 is a
graph of the input RMS ripple current, normalized
relative to output load current, as a function of duty
cycle that is adjusted for converter efficiency. The ripple
current calculation is written as Equation 41:
Where:
Duty cycle is written as Equation 42:
In addition to the bulk capacitors, some low ESL ceramic
capacitors are recommended to decouple between the
ΔV
I
D
IN_RMS
O
- I
- x is a multiplier (0 to 1) corresponding to the
- D is the duty cycle that is adjusted to take into
C
=
ESR
into which ripple current I
converter
inductor peak-to-peak ripple amplitude expressed
as a percentage of I
account the efficiency of the converter
------------------------- -
V
=
MAX
IN
-------------------------------- -
8 C
=
V
⋅
⋅
=
O
EFF
I
P-P
is the maximum continuous I
I
------------------------------------------------------------------------------------------------------- -
O
P-P
(
SW
⋅
I
⋅ SR
MAX
F
E
SW
. Take into account that the rated value of
2
⋅
(
D D
–
I
2
MAX
MAX
)
23
)
+
⎛
⎝
P-P
x
(0% to 100%)
2
⋅
ISL95870, ISL95870A, ISL95870B
I
can flow. Current I
MAX
2
⋅
----- -
12
D
LOAD
⎞
⎠
P-P
across C
of the
(EQ. 39)
(EQ. 40)
(EQ. 41)
(EQ. 42)
P-P
P-P
P-P
O,
drain of the high-side MOSFET and the source of the
low-side MOSFET.
FIGURE 19. NORMALIZED INPUT RMS CURRENT FOR
Selecting the Bootstrap Capacitor
The integrated driver features an internal bootstrap
schottky diode. Simply adding an external capacitor
across the BOOT and PHASE pins completes the
bootstrap circuit. The bootstrap capacitor voltage rating
is selected to be at least 10V. Although the theoretical
maximum voltage of the capacitor is PVCC-V
(voltage drop across the boot diode), large excursions
below ground by the phase node requires at least a 10V
rating for the bootstrap capacitor. The bootstrap
capacitor can be chosen from Equation 43:
Where:
As an example, suppose the high-side MOSFET has a
total gate charge Q
ΔV
is 0.125µF; for a comfortable margin, select a capacitor
that is double the calculated capacitance. In this
example, 0.22µF will suffice. Use a low
temperature-coefficient ceramic capacitor.
Driver Power Dissipation
Switching power dissipation in the driver is mainly a
function of the switching frequency and total gate charge
of the selected MOSFETs. Calculating the power
dissipation in the driver for a desired application is critical
to ensuring safe operation. Exceeding the maximum
allowable power dissipation level will push the IC beyond
the maximum recommended operating junction
temperature of +125°C. When designing the application,
it is recommended that the following calculation be
performed to ensure safe operation at the desired
frequency for the selected MOSFETs. The power
C
BOOT
- Q
- ΔV
BOOT
fully charge the gate of the upper MOSFET
capacitor
0.6
0.5
0.4
0.3
0.2
0.1
GATE
BOOT
0
≥
0
of 200mV. The calculated bootstrap capacitance
----------------------- -
ΔV
Q
GATE
is the amount of gate charge required to
0.1 0.2
BOOT
EFF = 1
is the maximum decay across the BOOT
x = 1
g
, of 25nC at V
0.3 0.4 0.5 0.6 0.7
x = 0.5
DUTY CYCLE
x = 0
GS
= 5V, and a
0.8 0.9
December 22, 2009
DIODE
(EQ. 43)
FN6899.0
1.0