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参数资料
| 型号： | ISL85402IRZ-TK |
| 厂商： | Intersil |
| 文件页数： | 15/22页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK BOOST SYNC ADJ 20QFN |
| 标准包装： | 1,000 |
| 类型： | 降压（降压），升压（升压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 可调至 0.8V |
| 输入电压： | 3 V ~ 36 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 200kHz ~ 2.2MHz |
| 电流 - 输出： | 2.5A |
| 同步整流器： | 两者兼有 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 20-VFQFN 裸露焊盘 |
| 包装： | 带卷 (TR) |
| 供应商设备封装： | 20-QFN（4x4） |
ISL85402
R UP ? 0.8
R LOW = ---------------------------------------
V OUTBST = V BAT + V OUT
P OUT (EQ. 8)
V BAT ? EFF
Use Equation 4 to calculate the lower resistor R LOW (R2 in Figure 26)
according to a desired boost enable threshold.
(EQ. 4)
VFTH – 0.8
Where VFTH is the desired falling threshold on boost input
voltage to turn on the boost, 3μA is the hysteresis current, and
0.8V is the reference voltage to be compared with.
Note the boost start-up threshold has to be selected in a way that
the buck is operating working well and kept in close loop
regulation before boost start-up. Otherwise, large in-rush current
at boost start-up could occur at boost input due to the buck open
loop saturation.
Similarly, a resistor divider from the boost output voltage to the
AUXVCC pin is used to detect the boost output voltage. When the
voltage on the AUXVCC pin is below 0.8V, the boost PWM is
enabled with a fixed 500μs soft-start, and a 3μA sinking current
is enabled at AUXVCC pin for hysteresis purposes. When the
voltage on the AUXVCC pin recovers to be above 0.8V, the boost
PWM is disabled immediately. Use Equation 3 to calculate the
upper resistor R UP (R 3 in Figure 26) according to a desired
hysteresis V HY at boost output voltage. Use Equation 4 to
calculate the lower resistor R LOW (R 4 in Figure 26) according to a
desired boost enable threshold at boost output.
Assuming V BAT is the boost input voltage, V OUTBST is the boost
output voltage and V OUT is the buck output voltage, the steady
state transfer function are:
From Equations 5 and 6, Equation 7 can be derived to estimate
the steady state boost output voltage as function of V BAT and
V OUT :
(EQ. 7)
After the IC starts up, the boost buck converters can keep
working when the battery voltage drops extremely low because
the IC’s bias (VCC) LDO is powered by the boost output. For
example, a 3.3V output application battery drops to 2V, and the
VIN pin voltage is powered by the boost output voltage that is
5.2V (Equation 7), meaning that the VIN pin (buck input) still sees
5.2V to keep the IC working.
Note that in the previously mentioned case, the boost input current
could be high because the input voltage is very low
(V IN *I IN = V OUT *I OUT /Efficiency). If the design is to achieve the low
input operation with full load, the inductor and MOSFET have to be
selected with enough current ratings to handle the high current
appearing at boost input. The boost inductor current are the same
with the boost input current, which can be estimated as Equation 8,
where P OUT is the output power, V BAT is the boost input voltage, and
EFF is the estimated efficiency of the whole boost and buck stages.
IL IN = --------------------------------------
Based on the same concerns of the boost input current, the IC
should be disabled before the boost input voltage rises above a
certain level. PFM is not available in boost mode.
V OUTBST = ------------------ ? V BAT
V OUT = D ? V OUTBST = ------------------ ? V BAT
1
1 – D
D
1 – D
(EQ. 5)
(EQ. 6)
Oscillator and Synchronization
The oscillator has a default frequency of 500kHz with the FS pin
connected to VCC, or ground, or floating. The frequency can be
programmed to any frequency between 200kHz and 2.2MHz with
a resistor from FS pin to GND.
R FS [ k Ω ] = ------------------------------------------------------------------------------------
BATTERY
145000 – 16 ? FS [ kHz ]
FS [ kHz ]
VOUT_BST
(EQ. 9)
R1
EXT_BOOST
+
+
R2
0.8V
I_HYS = 3μA
R3
R4
AUXVCC
0.8V
LOGIC
PWM
LGATE
DRIVE
LGATE
I_HYS = 3μA
FIGURE 26. BOOST CONVERTER CONTROL
15
FN7640.1
April 25, 2013
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