參數(shù)資料
型號(hào): ISL88550A
廠商: Intersil Corporation
英文描述: Synchronous Step Down Controller with Sourcing and Sinking LDO Regulator
中文描述: 同步降壓控制器,具有采購(gòu)和下沉LDO穩(wěn)壓器
文件頁(yè)數(shù): 23/25頁(yè)
文件大?。?/td> 928K
代理商: ISL88550A
23
FN6168.0
October 12, 2005
FIGURE 29. TYPICAL GFX APPLICATION CIRCUIT
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 T
ON
K-factor. This error is greater
at 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
SAG
equation in the Design
Procedure section).
The absolute point of dropout is when the inductor current
ramps down during the minimum off-time (I
DOWN
) as much
as it ramps up during the on-time (I
UP
). The ratio
h = I
UP
/I
DOWN
indicates the controller’s ability to slew the
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
SAG
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
SAG
, output
capacitance, and minimum operating voltage. For a given
value of h, the minimum operating voltage can be calculated
as:
1μF
100k
R3
C3
330μF
9m
OVP/UVP
VDD
VIN
BOOT
UGATE
PHASE
LGATE
PGND1
OUT
ILIM
REF
FB
REFIN
VTTR
VIN: 4.5V to 25V
GFXCORE
0.95V/12A
Q1
Q2
-
C8: 2x10μF
C11
0.22μF
ISL88550A
AVDD
5V BIAS SUPPLY
C5: 4.7μF
SS
TON
SKIP#
GND
STBY#
SHDNA#
POK2
POK1
VTTI
VTT
VTTS
PGND2
1.8V
C4
2x10μF
C6
1μF
C2
10μF
C12
330μF
9m
AVDD
100k
1V/10mA
0.22μF
1μF
Q1: IRF7821/30V/9m
Q2: IRF7832/30V/5m
C14
470μF
(OPTIONAL)
R4
200k
R5
56.2k
R1: 182k
L1:
FALCO ER1309
1.0μH, 35A, 2m
C7
PCI-e
1.2V/2A
R2
C9: OPEN
C10
C13
R6
24.9k
R7
69.8k
R8
69.8k
GPIO
GPIO OPEN : GFXCORE = 0.95V
GPIO LOW : GFXCORE = 1.20V
R9
1.21k
R10
4.99k
(
)
(
)
1
DROP
V
2
DROP
V
MIN
OFF
K
1
DROP
V
OUT
h
MIN
IN
t
1
V
V
+
×
+
=
ISL88550A
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