HT7936B datasheet(4/16 Pages) HOLTEK

4 / 16 page

HT7936A/HT7936B

Rev 1.00

December 28, 2009

Functional Description

These DC-DC step-up converters can generate fixed

level output voltages higher than their input supply volt-

age.

Operation

Utilising a charge pump method with three external ca-

pacitors to store and transfer energy, the devices can

transfer electrical energy from input to output at different

voltage levels. Since the capacitors are not able to

change their voltage level abruptly, the VOUT/VIN voltage

ratio of VOUT over VIN is limited within a fixed range. Ca-

pacitive voltage conversion is obtained by the periodic

switching of a capacitor. It first charges the capacitor

CPUMP by connecting it across a voltage source after

which it is connected to the output. Referring to Fig-

ure.1, during the on state of the internal clock, M1 and

M3 are closed, M2 and M4 are opened, which charges

CPUMP to a VIN level. During the off state of the internal

clock, M2 and M4 are closed, M1 and M3 are opened,

which transfers the energy to the output. The output

voltage is therefore VIN plus VCPUMP, which is in effect

equal to 2VIN.

Shutdown

The devices can be shutdown by setting the EN input

pin to a low level. In the shutdown mode, the output is

disconnected from the input. The input current will re-

duce to an extremely low level as most of the internal cir-

cuitry is inactive. As the EN pin is a high impedance

input, it must not be allowed to float.

Short Circuit Protection

The devices contain integrated short circuit protection

circuits with current limiting protection. During a short

circuit condition, the output current is automatically lim-

ited to a typical value of approximately 60mA. If the fault

does not clear itself, the protect operation will repeat

continuously. This protection feature allows the devices

to operate indefinitely under short circuit conditions

without damaging the device.

Capacitor Selection

Careful selection of the three external capacitors CIN,

COUT and CPUMP is important because they will affect

ramp-up time, output ripple and transient performance.

Optimum performance will be obtained when low ESR

(<100m

W) ceramic capacitors are used for CIN and

COUT and CPUMP.

In general, a low ESR may be defined as less than

100m

W. In all cases, X7R or X5R dielectrics are recom-

mended. For certain applications, low ESR Tantalum

capacitors may be substituted, however optimum output

ripple performance may not be attainable. Aluminum

electrolytic capacitors are not recommended for use

with these devices due to their inherent high ESR char-

acteristics.

In general the CIN,COUT and CPUMP capacitors may

range from 1

mFto10mF for heavy output load condi-

tions. If the CPUMP capacitor is increased, then COUT

should also be increased by the same ratio to minimise

output ripple. Lowering the values of CIN,COUT and

CPUMP may decrease the ramp-up time of VOUT, but it

will inversely increase the output ripple.

Efficiency

The efficiency of the charge pump regulator varies with

the output voltage type, the applied input voltage, the

load current and the internal operation mode of the de-

vice.

The approximate device efficiency is given by:

Due to internal switching losses and IC quiescent cur-

rent consumption, the actual measured efficiency will

decrease.

V O U T

C O U T

C I N

M 1

M 2

M 3

M 4

V I N

C P U M P

Figure.1

HT7936B Datasheet(PDF) 4 Page - Holtek Semiconductor Inc

HT7936B Datasheet(HTML) 4 Page - Holtek Semiconductor Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL