参数资料
型号: IRDC3856
厂商: International Rectifier
文件页数: 18/35页
文件大小: 0K
描述: BOARD EVAL FOR IR3856
标准包装: 1
系列: SupIRBuck™
主要目的: DC/DC,步降
输出及类型: 1,非隔离
输出电压: 1.8V
电流 - 输出: 0 ~ 6A
输入电压: 12V
稳压器拓扑结构: 降压
频率 - 开关: 600kHz
板类型: 完全填充
已供物品:
已用 IC / 零件: IR3856
PD-97528
IR3856MPbF
Minimum on time Considerations
The minimum ON time is the shortest amount of
time for which the Control FET may be reliably
turned on, and this depends on the internal
timing delays. For the IR3856, the typical
minimum on-time is specified as 50 ns.
Any design or application using the IR3856 must
ensure operation with a pulse width that is higher
than this minimum on-time and preferably higher
than 100 ns. This is necessary for the circuit to
operate without jitter and pulse-skipping, which
can cause high inductor current ripple and high
output voltage ripple.
Maximum Duty Ratio Considerations
A fixed off-time of 200 ns maximum is specified
for the IR3856. This provides an upper limit on
the operating duty ratio at any given switching
frequency. It is clear, that higher the switching
frequency, the lower is the maximum duty ratio at
which the IR3856 can operate. To allow a margin
of 50ns, the maximum operating duty ratio in any
application using the IR3856 should still
accommodate about 250 ns off-time. Fig 10.
shows a plot of the maximum duty ratio v/s the
switching frequency, with 250 ns off-time.
t on =
=
D
F s
V out
V in × F s
95
90
85
80
75
M a x Duty Cycle
In any application that uses the IR3856, the
following condition must be satisfied:
70
65
60
t on (min) ≤ t on
55
250
450
650
850
1050
1250
1450
1650
∴ t on (min) ≤
V out
V in × F s
S w itchin g Freq uency (kH z )
Fig. 11. Maximum duty cycle v/s switching
∴ V in × F s ≤
V out
t on (min)
frequency.
The minimum output voltage is limited by the
reference voltage and hence V out(min) = 0.7 V.
Therefore, for V out(min) = 0.7 V,
∴ V in × F s ≤
∴ V in × F s ≤
V out (min)
t on (min)
0.7 V
100 ns
= 7 × 10 6 V/s
Therefore, at the maximum recommended input
voltage 21V and minimum output voltage, the
converter should be designed at a switching
frequency that does not exceed 333 kHz.
Conversely, for operation at the maximum
recommended operating frequency 1.65 MHz
and minimum output voltage, any voltage above
4.2 V may not be stepped down without pulse-
skipping.
Rev 4.0
18
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