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Test Circuit
Applications Information
OVERVIEW
The LM2636 is a high speed synchronous PWM buck regu-
lator controller designed for VRM vendors or motherboard
manufacturers who need to build on-board power supplies
for Cyrix MII, Pentium II or Deschutes microprocessors. It
has a built-in 5-bit DAC to decode the 5-bit word provided by
the CPU and supply the corresponding voltage. It also has
the power good (PWRGD) and output enable (OUTEN) func-
tions required by the VRM specification. It employs a voltage
mode control scheme plus two fast responding comparators
to quickly respond to large load transients. It has two fast
FET drivers to drive the high-side and low-side NMOS
switches of a synchronous buck regulator. The PWM fre-
quency is adjustable from 50 kHz to 1 MHz through an
external resistor. Over-voltage protection is achieved by
shutting off the high-side driver and turning on the low-side
driver 100% of the time. Current limiting is implemented by
sensing V
DS of the high-side NMOS switch and shutting it off
for the present switching cycle when an over current condi-
tion is detected. Soft start functionality is realized through an
internal digital counter and an internal DAC.
THEORY OF OPERATION
Start Up
When V
CC voltage exceeds 4.2V, OUTEN pin is a logic high
and the VID code is valid, the soft start circuitry starts to
work. The duration of the soft start is determined by an
internal digital counter and the switching frequency. During
soft start, the output of the error amplifier is allowed to
increase gradually. When the counter has counted 2,048
clock cycles, the soft start session ends and the output
voltage level of the error amplifier is released and allowed to
go to a value that is determined by the feedback loop.
PWRGD pin is forced low during soft start and is turned over
to output voltage monitoring circuitry after that. Before V
CC
reaches 4.2V, all internal logic is in a power on reset state
and the two FET drivers are disabled.
During normal operation, if V
CC voltage drops below 3.8V,
the internal circuitry will go into power on reset again. The
hysteresis helps decrease the noise sensitivity on the V
CC
pin. After soft starts ends and during normal operation, if the
converter output voltage exceeds 115% of the DAC output
voltage, the LM2636 will lock into over voltage protection
mode. The high side drive will be disabled, and the low side
drive will be high. There are two ways to clear the mode. One
is to cycle V
CC voltage once. The other is to toggle the
OUTEN level. After the over voltage protection mode is
cleared, the LM2636 will enter the soft start session and start
over.
Normal Operation
During the normal operation mode, the LM2636 regulates
the converter output voltage by adjusting the duty ratio. The
output voltage is determined by the 5-bit VID code set by the
user/load.
The PWM frequency is set by the external resistor between
FREQ_ADJ pin and ground. The resistance needed for a
desired switching frequency is:
For example, if the desired switching frequency is 300 kHz,
the resistance should be around 84 k
.
The minimum allowable PWM frequency is 5 kHz.
MOSFET Gate Drive
The LM2636 has two gate drives that are suitable for driving
external N-MOSFETs in a synchronous buck topology. The
power for the two FET drivers is supplied by the BOOTV pin.
This BOOTV voltage needs to be at least one V
GS(th) higher
than the converter input voltage for the high side FET to be
10083404
FIGURE 2.
LM2636
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相关代理商/技术参数
LM2636MTC 制造商:OC White Company 功能描述:DC DC Cntrlr Single-OUT Sync Buck 4.5V to 5.5V Input 20-Pin TSSOP Rail
LM2636MTC/NOPB 功能描述:IC REG SYNCH BUCK 5-BIT 20-TSSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 电源管理 - 专用 系列:- 应用说明:Ultrasound Imaging Systems Application Note 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:37 系列:- 应用:医疗用超声波成像，声纳 电流 - 电源:- 电源电压:2.37 V ~ 6 V 工作温度:0°C ~ 70°C 安装类型:表面贴装 封装/外壳:56-WFQFN 裸露焊盘 供应商设备封装:56-TQFN-EP（8x8） 包装:管件
LM2636MTCX 制造商:未知厂家 制造商全称:未知厂家 功能描述:Voltage-Mode SMPS Controller
LM2636MTCX/NOPB 功能描述:IC CTRLR PROG REG 5BIT 20TSSOP RoHS:是 类别:集成电路 (IC) >> PMIC - 电源管理 - 专用 系列:- 应用说明:Ultrasound Imaging Systems Application Note 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:37 系列:- 应用:医疗用超声波成像，声纳 电流 - 电源:- 电源电压:2.37 V ~ 6 V 工作温度:0°C ~ 70°C 安装类型:表面贴装 封装/外壳:56-WFQFN 裸露焊盘 供应商设备封装:56-TQFN-EP（8x8） 包装:管件
LM2636MX 制造商:Texas Instruments 功能描述:
LM2636MX/NOPB 功能描述:IC CTRLR PROG REG 5BIT 20-SOIC RoHS:是 类别:集成电路 (IC) >> PMIC - 电源管理 - 专用 系列:- 应用说明:Ultrasound Imaging Systems Application Note 产品培训模块:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 标准包装:37 系列:- 应用:医疗用超声波成像，声纳 电流 - 电源:- 电源电压:2.37 V ~ 6 V 工作温度:0°C ~ 70°C 安装类型:表面贴装 封装/外壳:56-WFQFN 裸露焊盘 供应商设备封装:56-TQFN-EP（8x8） 包装:管件
LM2637 制造商:未知厂家 制造商全称:未知厂家 功能描述:
LM2637A E WAF 制造商:Texas Instruments 功能描述: