isl88550a Intersil Corporation, isl88550a Datasheet - Page 23

• Manufacturer Part Number: isl88550a
• Description: Synchronous Step Down Controller With Sourcing And Sinking Ldo Regulator
• Manufacturer: Intersil Corporation

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
Procedure” on page 17).
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
100k Ω
R3
PCI-e
1.2V/2A
2x10µF
AVDD
AVDD
C4
1.8V
t
10µF
ON
R2
100k Ω
4.99k Ω
C2
C2
1.21k Ω
1µF
C3
C9: OPEN
K-factor. This error is greater at
23
R10
R9
SAG
UP
equation in “Design
). 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
h = I
inductor current higher in response to increased load, and
must always be greater than 1. As h approaches 1, the
absolute minimum dropout point, the inductor current cannot
increase as much during each switching cycle and V
greatly 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
C6
1µF
C10
C10
0.22µF
V
IN
(
Q1
Q1
MIN
UP
)
=
/I
⎡
⎢
⎢
⎢
⎢
⎢
⎣
R1: 182k Ω
DROP1
DOWN
Q2
1
C5: 4.7µF
R5
56.2k Ω
−
V
C8: 2x10µF
1.0µH, 35A, 2m Ω
5V BIAS SUPPLY
FALCO ER1309
OUT
⎛ ×
⎜
⎜
⎝
h
24.9k Ω
+
t
indicates the controller’s ability to slew the
OFF
V
and V
K
R6
DROP
L1:
(
MIN
1
)
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
C11
9m Ω
330µF
R8
69.8k Ω
C14
470µF
(OPTIONAL)
are the parasitic voltage drops
2
−
V
DROP
C12
C12
330µF
9m Ω
1
GPIO
SAG
GFXCORE
0.95V/12A
, output
C13
C13
1µF
October 1, 2007
SAG
(EQ. 35)
FN6168.2