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参数资料
| 型号： | LTC3785EUF-1#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 9/20页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR BST PWM VM 24-QFN |
| 标准包装： | 91 |
| PWM 型： | 电压模式 |
| 输出数： | 2 |
| 频率 - 最大： | 1MHz |
| 占空比： | 99% |
| 电源电压： | 2.7 V ~ 10 V |
| 降压： | 是 |
| 升压： | 是 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 85°C |
| 封装/外壳： | 24-WFQFN 裸露焊盘 |
| 包装： | 管件 |
LTC3785-1
OPERATION
The input voltage, V IN , where the four switch region begins
is given by:
In Burst Mode operation the maximum output current is
given by:
V IN =
V OUT
1– ( 300ns ? f )
V
I OUT(MAX,BURST) ≈
1.2 ? V IN
f ? L ? ( V OUT + V IN )
A
t ON =
the point at which the four switch region ends is given
by:
V IN = V OUT (1 – D) = V OUT (1 – 300ns ? f) V
Boost Region (V IN < V OUT )
Switch A is always on and switch B is always off during
boost mode. When the error amp output voltage, V C , is ap-
proximately above 0.7V, switch pair C and D will alternately
switch to provide a boosted output voltage. This operation
is typical to a synchronous boost regulator. The maximum
duty cycle of the converter is limited to 90% typical.
Burst Mode OPERATION
During Burst Mode operation, the LTC3785-1 delivers
energy to the output until it is regulated and then goes
into a sleep state where the outputs are off and the IC
is consuming only 86μA. In Burst Mode operation, the
output ripple has a variable frequency component, which
is dependent upon load current
During the period where the converter is delivering en-
ergy to the output, the inductor will reach a peak current
determined by an on time, t ON , and will terminate at zero
current for each cycle. The on time is given by:
2.4
V IN ? f
where f is the oscillator frequency.
The peak current is given by:
Burst Mode operation is user-controlled by driving the
MODE pin high to enable and low to disable.
V CC REGULATOR
An internal P-channel low dropout regulator produces
4.35V at the V CC pin from the V IN supply pin. V CC powers
the drivers and internal circuitry of the LTC3785-1. The
V CC pin regulator can supply a peak current of 100mA and
must be bypassed to ground with a minimum of 4.7μF
placed directly adjacent to the V CC and GND pins. Good
bypassing is necessary to supply the high transient cur-
rent required by the MOSFET gate drivers and to prevent
interaction between channels. If desired, the V CC regulator
can be connected to V OUT through a Schottky diode to
provide higher gate drive in low input voltage applications.
The V CC regulator can also be driven with an external 5V
source directly (without a Schottky diode).
TOPSIDE MOSFET DRIVER SUPPLY (V BST1 , V BST2 )
The external bootstrap capacitors connected to the V BST1
and V BST2 pins supply the gate drive voltage for the top-
side MOSFET switches A and D. When the top MOSFET
switch A turns on, the switch node SW1 rises to V IN and
the V BST2 pin rises to approximately V IN + V CC . When the
bottom MOSFET switch B turns on, the switch node SW1
drops low and the boost capacitor is charged through the
diode connected to V CC . When the top MOSFET switch D
turns on, the switch node SW2 rises to V OUT and the V BST2
pin rises to approximately V OUT + V CC . When the bottom
I PEAK =
I PEAK =
V IN
L
2.4
f ?L
? t ON
MOSFET switch C turns on, the switch node SW2 drops
low and the boost capacitor is charged through the diode
connected to V CC . The boost capacitors need to store about
100 times the gate charge required by the top MOSFET
switch A and D. In most applications a 0.1μF to 0.47μF,
So the peak current is independent of V IN and inversely
X5R or X7R dielectric capacitor is adequate.
proportional to the f ? L product optimizing the energy
transfer for various applications.
37851fa
9
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