ISL88550AIRZ Intersil, ISL88550AIRZ Datasheet - Page 23

• Manufacturer Part Number: ISL88550AIRZ
• Description: IC PWM CONTROLLER 28TQFN
• Manufacturer: Intersil

Specifications of ISL88550AIRZ

• Applications: PWM Controller
• Voltage - Input: 2 ~ 25 V
• Current - Supply: 25µA
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN
• Rohs Compliant: YES
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Voltage - Supply: -

Applications Information
Dropout Performance (Buck)
The output voltage adjustable range for continuous
conduction operation is restricted by the non-adjustable
minimum off-time one-shot. For best dropout performance,
use the slower (200kHz) ON-time setting. When working
with low input voltages, the duty-factor limit must be
calculated using the worse case values for on and off times.
Manufacturing tolerances and internal propagation delays
introduce an error to the
higher frequencies (see Table 1). Also, keep in mind that
transient response performance of buck regulators operated
too close to dropout is poor, and bulk output capacitance
must often be added (see the V
Response (Buck)” on page 22).
The absolute point of dropout is when the inductor current
ramps down during the minimum off-time (I
as it ramps up during the on-time (I
h = I
UP
/I
DOWN
100k Ω
2µFx10µF
R3
indicates the controller’s ability to slew the
PCI-e
1.2V/2A
AVDD
AVDD
C4
1.8V
t
10µF
ON
R2
100k Ω
4.99k Ω
C2
1.21k Ω
1µF
C3
C9: OPEN
K-factor. This error is greater at
23
R10
R9
SAG
UP
equation in “Transient
). The ratio
AVDD
SS
TON
SKIP#
GND
STBY#
SHDNA#
POK2
POK1
VTTI
VTT
VTTS
PGND2
FIGURE 30. TYPICAL GFX APPLICATION CIRCUIT
DOWN
ISL88550A
) as much
OVP/UVP
UGATE
PHASE
PGND1
LGATE
REFIN
BOOT
VTTR
VDD
OUT
ILIM
REF
VIN
ISL88550A
FB
200k Ω
1V/10mA
R4
C7
C7
0.22µF
inductor current higher in response to increased load, and
must always be >1. As h approaches 1, the absolute
minimum dropout point, the inductor current cannot increase
as much during each switching cycle and V
increases, unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting this
up or down allows trade-offs between V
capacitance, and minimum operating voltage. For a given
value of h, the minimum operating voltage can be calculated
using Equation 35:
where V
in the discharge and charge paths (see “ON-Time One Shot
(t
Specifications” Table on page 3, and K is taken from Table 1.
The absolute minimum input voltage is calculated with h = 1.
C6
1µF
C10
C10
0.22µF
V
ON
IN
(
Q1
Q1
MIN
)” on page 12), t
)
=
⎡
⎢
⎢
⎢
⎢
⎢
⎣
R1: 182k Ω
DROP1
Q2
1
C5: 4.7µF
R5
56.2k Ω
−
V
C8: 2µFx10µF
1.0µH, 35A, 2m Ω
5V BIAS SUPPLY
FALCO ER1309
OUT
⎛ ×
⎜
⎜
⎝
h
24.9k Ω
+
t
OFF
V
and V
K
R6
DROP
L1:
(
MIN
1
OFF(MIN)
)
R7
69.8k Ω
⎞
⎟
⎟
⎠
DROP2
VIN: 4.5V TO 25V
⎤
⎥
⎥
⎥
⎥
⎥
⎦
-
GPIO OPEN : GFXCORE = 0.95V
GPIO LOW : GFXCORE = 1.20V
+
Q1: IRF7821/30V/9m Ω
Q2: IRF7832/30V/5m Ω
V
DROP
330µF
9m Ω
C11
R8
69.8k Ω
C14
470µF
(OPTIONAL)
are the parasitic voltage drops
2
is from the “Electrical
−
V
DROP
330µF
C12
9m Ω
1
GPIO
SAG
GFXCORE
0.95V/12A
SAG
, output
1µF
C13
greatly
April 23, 2008
(EQ. 35)
FN6168.3