LTC3700EMS View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

LTC3700EMS

Constant Frequency Step-Down DC/DC Controller with LDO Regulator

Linear Technology 

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OPERATIO (Refer to Functional Diagram)

when the buck is operating below 40% duty cycle. How-

ever, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak induc-

tor current. The amount of reduction is given by the curves

in Figure 2.

Soft-Start

An internal default soft-start circuit is employed at power

up and/or when coming out of shutdown. The soft-start

circuit works by internally clamping the voltage at the ITH/

RUN pin to the corresponding zero-current level and

gradually raising the clamp voltage such that the minimum

time required for the programmed switch current to reach

its maximum is approximately 0.5msec. After the soft-

start circuit has timed out, it is disabled until the part is put

in shutdown again or the input supply is cycled.

LDO Regulator

The 150mA low dropout (LDO) regulator on the LTC3700

employs an internal P-channel MOSFET pass device be-

tween its input supply (VIN2) and the LDO output pin. The

pass FET has an on-resistance of approximately 1.5Ω

(with VIN2 = 4.2V) with a strong dependence on input

supply voltage. The dropout voltage is simply the FET on-

resistance multiplied by the load current when in dropout.

LTC3700

The LDO is protected by both current limit and thermal

shutdown circuits. Current limit is set such that the output

voltage will start dropping out when the load current reaches

approximately 200mA. With a short-circuited LDO output,

the device will limit the sourced current to approximately

225mA. The thermal shutdown circuit has a typical trip

point of 150°C with a typical hysteresis of 5°C. In thermal

shutdown, the LDO pass device is turned off.

Frequency compensation of the LDO is accomplished by

forcing the dominant pole at the output. For stability, a low

ESR ceramic capacitor ≥ 2.2µF is required from LDO to

GND. For improved transient response, particularly at

heavy loads, it is recommended to use the largest value of

capacitor available in the same size considered.

Both the buck and the LDO share the same internally

generated bandgap reference voltage for their feedback

reference. When both input supplies are present, the

internal reference is powered by the buck input supply

(VIN). For this reason, line regulation for the LDO output is

specified both with respect to VIN and VIN2 if the buck is

present and with respect only to VIN2 if the buck is

disabled. The same is true for VIN2 supply current, which

will be higher when the buck is disabled by the current

draw of the internal reference.

110

100

90

80

70

60

50

40

IRIPPLE = 0.4IPK

AT 5% DUTY CYCLE

30

IRIPPLE = 0.2IPK

AT 5% DUTY CYCLE

20

VIN = 4.2V

10

0 10 20 30 40 50 60 70 80 90 100

DUTY CYCLE (%)

3700 F02

Figure 2. Maximum Output Current vs Duty Cycle

3700f

9

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